Yes - your three times table is a good guide. If you're not great with mental arithmetic though, memorising a couple of descent gates works pretty much as well:
270 knots, 9000 feet, 30 miles
250 knots, 5000 feet, 20 miles
180 knots, 3000 feet 10 miles
And yes, the problem I see the most in new pilots is a fear of being too fast, and then being rushed into the approach. They tend to over-correct for that, and want to slow down to, say 220 knots at 20 miles or more. Now this is much closer to an efficient glide speed (green dot) than 250 knots is, at 220 knots the aircraft isn't washing off energy in parasite drag the way it would be at 250 knots ...so the aircraft drifts high on the approach path. Now the pilot has TWO problems to deal with - getting back onto the approach path AND dealing with their excess energy.
It is far better to be fast, and on the approach path, than it is to be high and on speed. If you are fast and you can intercept an ILS glide path (or stay in VNAV) then the aircraft autopilot will keep you on the correct slope, and you ONLY have to deal with the speed - and that's easy. Put the gear down, if you need it. You can easliy be 220 knots at 10 miles, on glide path (or 210 at 9 miles, 200 at 8 miles, 190 at 7 miles, 180 at six miles) and putting the gear down will have you stable at 1000 feet.
In setting up for the approach (deciding when to slow down) the aircraft (WITHOUT touching speed brakes) will slow down in Open Descent 10 knots every 1 mile, and will do that in 2/3 rds the distance if you are level. Say you are on vectors, level at 5000 feet on downwind at 250 kts. You know the controller will turn you onto base at ten miles for the ILS, when do you have to slow for that? Well if you want to be 220kts turning base, then you know you'll only take (250 kts - 220 kts =) 3 miles to lose that speed if you are in open descent, and 2 miles if you are level. You can keep that 250 kts to about 5 miles past the field, select 220 and you'll be at 220 for the base turn, take Flap One and 180 kts turning base an you'll slow to 180 and descend to 3000' for the ILS on base no problems - and that's without going for speedbrake or gear which is always available if you get turned early or run into a tailwind on base or approach.
Good luck with it all. It comes in time.
Last edited by Checkboard; 12th Jun 2023 at 15:10.