The 3 times formula works just fine for the major part of the descent at high speed, but it does not allow for the 2 major decelerations needed during descent, i.e. (1) the deceleration to 250 KIAS at 10000 ft / Terminal Area entry, and (2) later deceleration to approach speed.
Try using 3 X Ht +10 for TOD. The +10 makes good allowance for these 2 decelerations which "stretch" your glide. After the decelation has been made, use a simple "3 times".
This works pretty well with aircraft such as the MD-80 which are pretty good at slowing down. Others like the B777 are more reluctant to slow down, and a +20 additive is more appropriate. I'm sure that by experimentation you'll find the appropriate additive for your operation.
Regards,
Old Smokey