Typically, manways are "pressure rated components" i.e. qualified by test.
When an ASME vessel is fabicated, the vendor is responsible to perform calculations on the heads, shell, nozzles etc, However, pressure rated components, such as manways are purchased with a special ASME stamp that verified that it is acceptable for service.
Reinforcement pad thickness and width need to be calculated because standard manway can't have enough thickness for all vessels on which it installed. Reinforcement calculation can be done the same way as circular nozzle taking long diameter of the manway as diameter of nozzle.
Additionally, if you want to do very tight calculation, you can calculate for each direction separately.
Reinforcement area A=d x tr x F for any direction where
d = opening size in the direction
tr = required thickness of shell,head,..
F = factor for steress diference by directions
Use 1 if you are not sure. See UG-37 for details.
Above formula need to be modified if material's allowable stress are different between shell, nozzle, repad.
