None of the comments that I've seen here reflect real world construction standards. Stud walls are made from 2x4s that are actually 3 and 5/8 in wide, a layer of 1/2-in drywall on each side results in a finished wall that is 4 and 5/8 thick. The header beam over the doorway should be at least a 2x6 which means they're solid wood from 1/2 in. up from the bottom edge that extends upward 5 and 5/8 in. Some conditions require a 2x6 wall stud with corresponding increases in size for the header. Usually the addition of drywall corner bead creates an out of plane condition whereby the corner bead extends a little bit past the flat face of the drywall. Doorways that have actual door jambs installed do not have corner bead, The trim around the door jamb lands flat on the wall. Because you have corner beads any trim you add will angle out away from the wall a little bit. If it were me I would find a piece of deck board which measures actual 1 in thick and screw that up to the header, making sure my deck board was a little bit wider than the opening which I presume to be 4 and 5/8 in.