anyewo
There are a few different ways and in the end they all work...it's merely personal preference- some favour more complicated formula to give it all an air of mystery.
Very simply in most aircraft,
3 x height to lose gives you distance (Track miles) to lose it + add a buffer for deceleration...say 5nm to slow down to 250 Kts at 3000m and another 5nm to slow down to initial configuration speed by the Initial Approach Fix.
Big winds aloft will require allowances to but even God doesn't have a rule of thumb for those
If all of a sudden you get track shortening just increase ROD by a 'suitable' amount - experience being the best guide- until you regain the original 3x profile plus buffers as appropriate.
You must keep track of the profile in your head every 1000' or so to pick trends early - before variations from your 'ideal' become so great that massive rates of descent are required - lowering gear early/ speed brake, etc or at the other extreme flying level spooled up and burning extra fuel. The later is not dangerous but is not terribly professional either - not the sign of a Commander who is on top of his craft.
If you are being radar vectored to the FAF or some other point keep the same running tally in your head of approximate track miles to run and compare it to your ideal profile...once again experience and practice will make this easy - familiarity with particular aerodromes/ATC will give you a good idea of how much you will be vectored on average & therefore how much to pad your descent point to allow for it.
As a general rule I always like to be a little high on profile in these cases...it nearly always works out better that way.
If you are being vectored around to the far side of the airfield for an appoach think of it as a big DME ARC and by estimating an 'average' DME distance you can estimate the track miles via the 1 in 60 rule- i.e. Initial track inbound 270 and being vectored to runway 06 via south of the field. That's 150 degrees - if they start vectoring you at say 20nm East of the field you might work on an average of 15nm away from the field the whole way around. Track miles in that vectoring would be about 36nm...approximately 12000' 'allowance'. From the same inbound track around to runway 36, average distance from field 10nm would be 1/6th of 90 degrees - 15 track miles/ 5000' altitude allowance
It's just practice...even if it's not your sector have it all worked out in your head where YOU would descend and keep a track all the way down. Pay attention to trends/habits of the various ATC at your regular destinations and you will pick up constant 'habits' of the local controllers and be able to allow for that in your descent planning...it's fun to outsmart their, completely innocent, attempts to spoil your smooth, professional descents
Remember always think in track miles to go to the distance/altitude point that is the end of your enroute descent and the beginning of final descent, generally about 10 DME/2500'. Always plan backwards from that point to your TOPD and allow as per above.
example:
Cruising FL 390. Inbound via 240 radial to a 10nm arc to runway 36 ILS/DME. Requirement to be at 210kts/Flap 10 at FAF (2500'/ 8 DME) 39-2.5 = 36.5 x 3 = 110 +10 (decel) = 120 track miles before FAF. Track miles in DME arc = 10 so still air TOPD = 110 DME.
If at 60 DME you get a vector direct FAF? If on profile you'll be at about 20000'.
20-2.5= 17.5 x 3 = 52 + 10 decel and + 8 because the FAF where you want to be at 2500'/210kts is before the field.
You need 70 and you only have about 52 to the FAF...only 42 allowing for deceleration.
Average Ground speed 300kts = 5nm/min which = 8+ minutes to lose 17500' which means 2200'/min + ROD required.
Or another way approximately 2.5nm/1000'. That's a 2.5x profile as opposed to your 'ideal' 3x ! 5nm/min devided by 2.5nm/1000' = 2200'/min ROD at LEAST required to regain profile AT the FAF.
Normal 3x descent profile at 330kts might yield a ROD of 1900'/min so to arrive AT the FAF configured for final approach you need to increase ROD by 300 odd FPM...increase by 1000'/min and you will regain your original profile at perhaps 10 or 15 nm before FAF and then you can resume normal profile ROD and arrive at the FAF well rested and not stressed
With practice and experience the VNAV function in the FMS will be;
1/. Treated with the caution bordering on contempt it deserves,
2/. Double checked for silly errors that can creep in when programming
3/. Happily ignored and not missed when the inevitable vectoring or late runway change turns the VNAV calculated TOPD into pure fiction.
I know it sounds hard but with lots of practice the brain can do all that last minute replaning in not much longer than it takes to turn onto the new heading direct to the FAF.
Chuck.