I have tested them and have one for shooters to test out...
The idea is simple and effective set up properly...
Back in the day we did the same thing just utilizing our pins...
The bows were slower and had a bigger pin gap...
However if on an average Alberta deer ...if 20yd pin was on top of back and 30yd pin was on bottom of belly we knew instantly the deer was 30 yds
Same could be done with moose, elk etc
We also found if all pins were in vitals...you were too dang close...lol
Even with smaller pin gap of todays bows one can still do the same for example... on a bigger full size 3D deer like we have in Alberta...the 20yd pin is on top of back and say 40 yd pin was on bottom of belly we would know deer is at 40 yds...of course this will vary with every shooter due to speed and set up!!!
The Dead on has a caculator for set up on all big game in North America...
For example a mature buck in Alberta is twice as thick/big as a mature buck in Carolina USA...so the caculator gives you referance for where you live...and its close...
I had one installed on a sight a couple years ago...with that set up it was scary close and instant...
Now that you mentioned it I think I will install one on present set up this spring...
Its inexspensive so you dont have much to lose by trying it!!
Neil