I've tested it and it doesnt seem to work properly.

I set the slider to 0% and promoted my rookies knowing what their base stats should be (like hp should go up from 5 to 6) and some of my soldiers recieved reduced stats.

The slider was on 0% randomness so it should stop Hidden potential from doing anything at all, sadly this isnt the case. I tried reloading the test save I have and ranking them up with the slider set at different percents. They never recieved any stat gains over vanilla only heavy stat reductions.

I presume this is broken.

hmmmm...i was hoping for a definitive answer from the author or another modder. guess we'll have to confirm Twitchz' assumptions ourselves