Having it always be 100% chance is pretty easy, it's just one line at Scene_Battle 2, line 113. Change it to "success = rand(100) < 50 * 3", and voila, they will always escape if they're allowed to.
OR
If you want it to be percent chance, change that line to "success = rand(100) < $game_variables[5]" with the variable 5 being replaced by the ID of what variable you want to use. It'll select a random number from 0-100, and if that number is lower than your percent variable, you can escape. I hope that's what you meant, anyway.