Why can't they go as near as they can get to 20/20 with one surgical procedure then, do it a second time to correct it completely?

Target: The desired vision per each patient (usually 20/20) is always targeted. In nearly all cases that is achievable with today's techniques. Typically, the higher the initial prescription, the more likely there will be a second treatment to correct the residual. On occasion, a small amount of prescription is remaining and a shorter treatment many times can correct it.