Rule of thumb to calculate " distance to descent "

[chfeudpail]

Hi,

This is my first post.

I would like to know what kind of "rule of thumb" you use to compute the distance to start a descent from a flight level at +- M.78, deceleration included. E.g. : FL330 M.78 , you want to descent to the ground : 0foot.

I would be pleased if you can explain me how you crosscheck you distance/altitude during the descent.

Thanks,

Chfeudpail