Assume our opponent opens x% of SBs but folds 100% to a 3bet (if he folds 100% the % he opens doesn't matter right?)

If he folds 100% of his opens and is never limping then obviously a winning strategy is to 3bet 100%. However, is this optimal? It makes sense that we should flat AA/KK hands because we would win at a higher rate than his raise/fold, but what % of hands do we think we make more by flatting than 3 betting? It's hard to get an exact % because both our skill level and his post flop skill level come into effect but roughly do we think flatting top 5% is good, top 20% or what kind of ball park figure?