One method that seems to be used often is as follows (giving your TOD as a distance from destination/specified point):
Altitude to loose (in tens) multiplied by 3, + 10 for deceleration.
e.g. 33,000 ft cruise, apt elev 3000 ft.
= 30 x 3 + 10
= 100 nm.
i.e. start your descent 100 nm from desination/point.
Use caution though as this may not be accurate enough for your ATPL studies.