You roll the dice. You take your chances
Honestly, the number shown when targeting is not the actual percentage it is a close approximation. It also only shows the chances for Weapon 1. Which for infantry is a rifle generally. It doesn't show odds for the squad LMG, or Squad Markman, or Squad Grenade Launcher, etc, etc.
As for the Sniper, never fire more than 2 shots per turn from him and set his range back to 1 hex when your done. This way he won't OpFire during Player Twos Turn unless he gets shot. Never fire and move in the same turn.
Scouts same as Sniper except maybe never shoot with them at all. Scouts are great scouting. They kind of suck at the whole fighting thing.
The designers have for the longest time tried to make things "uncertain". They do a wonderful job of it and I applaud them for it.
PS: Sorry to hear about your dead snipers.