For me, it would be easier to get to consistent thickness now rather than try to angle tenon shoulders. Others may disagree. I'd pick the flattest side and use it to register a marking gauge against to create my target thickness gauge line on the opposite side. The leg can't be that wide and, at 3' long (and a tapering surface at that), you should be able to remove 1/8" - 3/16" thickness in a reasonable amount of time. Suppose you could spot target the (too thick) area where the stretcher will tie in to the leg and concentrate on getting a consistent thickness at the tie-in spot only as opposed to the entire 3' length.