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
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.