I'm chopping the last mortise for the kitchen table. The mortise front wall needs to be, maybe not a perfect 9 1/2 degrees but it should be pretty damn close. As with all angled mortises a chisel guide helps maintain the angle.
Here is mine. I used the wedge to set the table saw to cut the guide that way the angle doesn't matter.
In the photo is my new favorite mortise chisel for mortises that are too wide to use the 1/2" pig sticker. The Narex firmer chisel has a Hornbeam handle which will work with a metal hammer, I'm not a fan of wood mallets or soft faced hammers for mortise work. The cutter while not great is good enough.