I've never seen the sneak/spot situation as requiring opposed rolls, but that may be because I never upgraded from RQ2 to RQ3...
What I would do is ask for a straight roll from the sneaker. If he fails he can be spotted by anyone looking in the right direction. If he succeds it takes a succesful Spot roll to spot him, and if he scores a critical success it takes a critical Spot to discover him.
If you want skill levels over 100% to make a difference, then borrow from the (optional?) combat rules: Any attack percentage over 100% acts as a penalty to parry, such as if you attack with a skill of 120% then the defender gets a -20% penalty to his parry skill.
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