I bore the half dovetail to the shortest length, clean that out and then rip the slopes with a hand saw, two cuts on the sides of the mortice. Use the trusty corner chisel to cut V-cuts and finish with a chisel making sure the top and bottom contact, You want this slope to be as flat as you can get it. If you are using a wedge, which you should, you will have a slope on both ends of the mortice. Take care on the wedge side where the wedge protrudes from the timber to ease the exit face, for if you don't you risk lifting material off the timber as the wedge tightens. That may not be clear until you see it happen.