There is something that is a potential problem I came across, sometime, the specific point you press with your mouse gives bad chance, and if you click on a different point on the same enemy with the same attack you get much better one.

Larian's Biggest Oversight, what to do about it, and My personal review of BG3 EA
"74.85% of you stood with the Tieflings, and 25.15% of you sided with Minthara. Good outweighs evil, it seems."