Sadly, the answer to this question is complex and not at all possible in one line, but the nutshell is simple, the top line in teh fuel map is mapped for the amount of boost you plan to run. If its mapped for say, 20, it will still access it at 14.7 and be slightly rich until teh boost reaches 20.

Its no different to running 38psi on a 3bar map sensor.

Incidentally, if you run a 3bar map sensor on a std 2bar mapped chip, say goodbye to your pistons in teh very near future.