Descent planning
Cosmos's advice is excellent: Watch the gadget do it. If you're learning to fly a new airplane, take a lesson from the autopilot. My recollection is that the VNAV feature on the generation of equipment that I flew (767) could get you in trouble on some arrivals if you didn't watch it close (I've been retired for 10 years). I remember the automation computing a TOD a little closer than I would do it manually with the 3X linear rule of thumb. Maybe the VNAV knew it could get more descent rate initially in thinner air but it seemed to get behind the schedule in the lower altitudes. If you're heavy; within say 20% of your max landing weight, add about 10 miles to your plan. If you have an above average tailwind, add another 5 miles and of course the appropriate mileage for mandatory and assigned speed reductions; usually 1nm/10kts. At any rate, on average, the 4X=1500fpm, 3X=2500fpm, 2X=3500fpm rule of thumb works well under nominal conditions. The point here; say you're using the 3X rule (1,000'/3miles) and you're not seeing a nominal 2500 fpm you're probably getting behind your plan. Of course all the above assumes you're descending at cruise Mach to some predetermined IAS (310-320+kts). At the lower altitudes and at 250kts you should see around 1800'/minunte. This is typically where you'll need to add some drag. In the terminal areas and on vectors, the controllers in Germany were pretty good at giving you a planned mileage to the platform altitude, but then they started messing with your speed. If I could, I'd have the final approach fix up on the "legs" page (Honeywell FMS) to have, at least, a linear mileage from which to plan.
Hope this does you some good. I'm sure everyone has a personal technique that has served them well.