another quick way is to use a sheet of plywood, etc for a jig. it has to be square on 2 sides- like factory edges, at 90*. measure down one leg your rise, the other side measure your run. set 2x4 on top of plywood, hold to marks,then draw a line on the 2x4 from the bottom side of the plywood. doing this would allow you to move the support around until you get the right dog ears you want.
poor resemblance, but the angled bit would be your support, top left corner must be 90*-
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squaring off the bottom end and setting it on a 2x ledger screwed to the wall would help with the strength, especially if the walls are finished so that you cant just sister em to a stud. do make sure they land on a stud, or they could push back and thru sheetrock.