I can see the reasoning behind your change but I feel it will have some ramifications that will change the game for the worse. It seems you feel the same changes are for the better. But in the end, it's your mod

Would I like for there to be reduced luck with regards to the countersnipes? Yes. But I feel that 50% is the best for 1v1 sniping duels. Anything else and I predict it will change the gameplay. Multiple snipers is the only thing that needs adressing imo, and it has been. If a person loses due to a countersnipe - when there are only 2 snipers in total on the field and one is on each side - then it is my opinion that he, most of the time, has already screwed up.
You say he deserves the countersnipe because it is skilled for him to get in position. I say he is unskilled for having played himself into a corner where he needs that 50% chance to win (he should have teched/capped/used his units differently etc. to make the situation less impactful on his chance to win).