You don't need much - the OGE hover ceiling vs gross weight chart in the POH is the place to start having worked out your pressure altitude and expected gross weight.
You need at least OGE performance and ideally some extra - unfortunately there are no graphs to tell you how much extra you have got.
Exactly right.
Hot and High is correct that the MTOW isn't a power limit but taking 5 or 10% off the weight would guarantee you a thrust margin of probably more than you need if the weight line is the first limit you hit rather than the OAT or Pressure altitude.
Those margins simply don't exist in the real world of Robinson helicopter operations. R22 useful load is 490lb. With two 170lb people inside that's less than full tanks, or less than two hours endurance. Raven II useful load is already discussed below and is equally challenging with 4 people and stuff on board and a desire to get real work done. If you are in a Raven I, or a fat pig of an R44 like a Clipper II (aka Raven II with pop-outs) with air conditioning, now you have lost 100lbs of useful load. The bottom line is that these machines are nearly always flying at near max. gross when full.
The only time you have the sort of margin you are talking about is when you are flying an R44 with only two people on board (or an R22 with one). If you want a "rocket ship", two people and half tanks. And the performance difference is dramatic. That makes for safer initial training, but ultimately one must learn to fly the machine the way it will really be used, and that means at max. weight. In the R22 that happens by default. In the R44, at least at the school I attend, that happens by finding two other erstwhile individuals to fill the back seats and endure max. performance takeoffs, confined space landings, and ham handed auto's without puking (screaming is allowed). Normally these are more students and it's actually quite a bit of fun as each student rotates through the pilot's seat. The repartee meter does get pegged!
Robinson marketed the R44 Cadet to address this issue, but it's just such a dumb choice on so many levels. Few small schools will buy one because they need a machine they can use for more than just training. And it actually offers too much performance and does not prepare one for the real world of a regular 44 loaded to the gills.
If you have the performance on the chart then you just need to confirm it before committing to landing - I'm sure you check your hover MAP against the placard to make sure it is correct before transitioning.
If you mean check your hover power vs. the 5-minute max. on the placard, of course. That is (should be) taught and is a no-brainer. It's generally taught that if you've got 2" MP below 5 minute max. you can make a max. performance takeoff. This was well and thoroughly taught to me, with innumerable exercises involving artificial power limits. During my private and commercial training there was a lot of emphasis placed on "making it out safely", including such things as shuttling partial loads to a spot where a max. perf. was not necessary and other techniques.
Once in the vicinity of the landing area, pull to your calculated max take off MAP - ie your max continuous plus 2.8 to make sure it is available without Nr decay or exceeding any other limits.
Actually the landing power check, if you even need one (you did plan it, right?), is generally as follows: assume level flight 500ft above the landing altitude at Vy, then see if you've got 6 or 8" of MP margin (depending on who you talk to) against 5 minute power (don't forget to check OAT, pressure altitude and the placard!) If you do then you are probably OK for OGE hover. This is poorly taught in my experience. At least it was where I learned. We did spend a lot of time exploring the lack of OGE hover using artificial power limits in a run-on landing environment, but never for spot landings, which I felt was an unfortunate hole in the instructional repertoire. I have since developed that experience after the commercial check-ride, but I feel it should have been sooner. In the school's defense it can be difficult to train if you don't live in a hot/high environment, and the potential for a training accident is high.
One problem is that the OGE graph is based on 5 min take off power and not the max continuous MAP which may pass people by if they are not paying attention - the problem being that if you need 5 min power to achieve OGE, you have no spare to deal with turbulence or any unexpected rate of descent - hence my advice - especially if you are hot and high, to have a thrust margin.
Welcome to the wonderful world of very light helicopters where those margins are razor thin if they exist at all.
Now a 5% thrust margin was defined in various military Operating Data Manuals (same as POH) as sufficient to overcome light turbulence or manoeuvre gently in the hover - in a couple it quantified a vertical rate of climb of between 100 and 200 ft/min so it is not a huge amount of excess power.
Clearly such advice doesn't exist in the POH but you could experiment to see what difference 1 inch extra MAP gives you from an OGE hover - it will at least give you an idea of the difference between Sea Level at plus 23 degrees and 5300' at plus 23 degrees since the max continuous MAPS are only about 1.4 ins apart from the placard.
Again we have the rules of thumb for MP margins as discussed above, that are admittedly not in the POH, to go by. They have proven themselves over the many decades that Robinson helicopters have been in service.
Regarding confined space operations: I can't speak to other schools. The school I use is a pretty good school in this regard. Like any school they are obviously focused on getting people to pass the FAA checkride. But they will train you just as hard and thoroughly as you can safely be trained. Not every student learns at the same rate, not every student wants to be an accomplished helicopter pilot (e.g. wealthy guy in a hurry--just make me good enough--one major reason for the Robinson accident rate). But for folks who have the drive to train more comprehensively, this place will take you way, way beyond paved runway to paved runway and Farmer Jone's field. During my commercial training surprise landings were as common as surprise auto's. Instructor says "See that hole? Put me in there. See that pinnacle? Put me on top." One of the more challenging sites is a mountaintop transmitter site that is very popular with the instructors. It's a forest of guy wires and tall pine trees and a 15ft square gravel pad on a jaunty slope. Really great stuff.
One does see quite a bit of this sort of thing on the schools who like to publish on Youtube. Mischa Gelb's stuff, for example. One would like to think that all schools are teaching the same stuff to the same level, but of course that is not true.