As a rule of thumb, this works quite nice:
For a 3° descent path angle, subtract your target altitude from your current altitude, drop the zeros and multiply by 3.
If you're flying at 27000 ft and need to descent to 3000 ft, this would be
27000 - 3000 = 24000
drop the zeros = 24
24 x 3 = 72
So you start your descent 72 nm before your target waypoint. (you can always add a few miles for safety)
To figure out what vertical speed you need roughly for the 3° angle:
Multiply your groundspeed in kts by 5.
280 kts would make a 1400 ft vertical speed.
Quite easy and works for any plane.