Like the guys say:
Height-to-be-lost (in thousands) x 3 (+ a couple NM buffer) = distance from TOD to fix. ROD = 5 x GS. Works perfectly every time.
E.g:
From 25.000' to 5.000' = 20.000' to be lost. Start descend some 60NM out. Buffer of 2-5NM optional, but recommended if you need to be on target speed when reaching the desired altitude.
If your GS is 300kt, use 1500'/min ROD. Needs to be re-calculated as GS increases in descend, and the wind component is likely to change.