Wot?
reddo and
Checkers sober? Quick! Someone notify the press!
I suspect that you're flying something like a Kingair,
anyewo. I fly a B200 Super Kingair, so the following might be helpful to you. For a standard profile, I use a "3 times" profile until within 5 NM, if on a visual approach with no ATC restrictions, or until GP intercept if in cloud. The way this works is that I work out the total amount of height to lose, to reach the runway, divide by 1000, then multiply by 3. This gives me the distance that I will use in the descent, as in the following example.
Descent from FL 240 to 1000 feet means I must descend thru a total of 23,000 feet.
Now divide 23,000 by 1000 = 23
Now multiply 23 by 3 = 69.
I now know that my descent requires 69 NM. But, as I want to make final configuration change at 5 NM, I add that on, so 69 + 5 = 74 NM. To achieve my "3 times" profile, I initially set up a ROD of 1200 FPM and allow the speed to build up to 220 KIAS.
If I then have to adjust my descent profile due to an ATC requirement that reduces the track distance available for my descent, I'll reduce power initially to 400 ft/lbs, increase ROD to 2,000 FPM and bleed the speed back to 170 KIAS, if icing is likely to be a problem, or 140 KIAS otherwise.
This gives me time to work out what my actual descent gradient needs to be, at the reduced track distance. For this, using your example and mine, I need 69 NM for the descent, but only have 44 NM available (your 25 NM short-cut). I must now lose 23,000 feet in 44 NM, which equates to about 500 FT/NM, which is close enough to 1500 FPM at 180 KTS G/S.
I then roughly figure my TAS and the wind and, mostly, it works out for my speed of 140 knots, so I can reduce my ROD from 2,000 FPM to 1500 FPM. If I must maintain 170 KIAS due to the possibility of icing, I'll hold that speed and my 2000 FPM ROD until I'm out of icing conditions - subject to any other ATC restrictions. In either case, at the reduced speed, I don't need to consider the 5 NM buffer for final configuration changes.
The operating environment plays a part in all of this. I can usually get away with a lot of very rough calculations because all my operations are in tropical areas. In other parts of the world, where the weather can be a more serious consideration, it would pay you to think a little more carefully about the above calculations.
After you've flown a few hundred hours in command and been given a few tricky clearances, you really DO get a feel for what you need to do by way of descent performance. This is especially true if you're very familiar with the place you're going to! The really good thing about a Kingair is that it's flexible enough to meet most challenges.