Max credit does not have anything to do with the credit your line gets...
it only helps decide which pairing(s) you get when more than one pairing exists in the layer at your seniority level... for example if your layer 1 consists of the following 4 pairings for the same week, and assuming all 4 pairings are available it will do the following...
if pairing 1 is a 4 day worth 20 hours
and pairing 2 and 3 are 2 day back to back worth 12 hours each
and pairing 4 is a 5 day worth 23.5
max credit will chose the 2 day back to backs..

It has nothing to do with line credit, if you want max line credit you have to specify a minimum in layers 1-6... I don't recommend putting a minimum in layer 7 because you run the risk of PBS starting over and dumping your higher layer pairings in favor of higher credit lower layer pairings.