Do most people use the belted mag go gage, which essentially sets the headspace for the belt width dimension? Or, use a fired case - then how would you do this the first time?

also, if using a case, some options would be

unfired - probably not worth discussing
fired and not resized or neck resized
fired and resized with the shoulder set back slightly
fired and resized firmly to the FL sizing die

Pros and cons of these please...