Aviation Rules of Thumb

Divide MDA (minimum descent altitude, in feet AGL) by 300, then add or subtract as necessary to find the DME point to start the descent. If final approach course is going away from the VOR, subtract from the DME MAP (missed approach point), if it is going towards the VOR, then add.

example: VOR/DME Approach with the FAF (final approach fix) over the VOR and MAP @ 4.5 DME. MDA is 400 ft AGL (400/300=1.3) Start descent @ 3.2 DME for a 3 degree glideslope.