Simple:

Altitude to lose multiplied by 3 then drop the zeros.

FL330 down to 12,000= 21kft x 3 = 63miles. Can add five miles for t/d start and level off.

How fast to descend? Again, simple: Grounds speeddivided by 2 add a zero.

400kts grndsp = 2000ft/m VSI. ( can use IAS, if needed, just recheck your math every so often to verify your descent).