i know i wouldnt mind something like that... but how beginner friendly would it be?
like keep it simple and everything otherwise it gets really confusing...
if you make it certain weapons always have a % then yeah why not but the bigger question is...
how do we deal with visibility and how does that affect the %
im not a active xcom player but i did play a fair share of games that use % based on vision

i would say something simpler... what about dice rolls...?
alien rolls a die you roll a die if your roll is higher than alien then alien gets hit
perhaps add modifier depending on position and weapon... say snipers have a +2 modifier and shotguns have a -4 on long distance