Typically, no, you shouldn't mount the sub to the seat frame. You should mount it to the floor with the proper backing plate. What people wind up doing is mounting the sub belt so that it's far enough back for all drivers and leave it at that.
http://www.schrothracing.com/docs/Competition_Instructions.pdf
Looking over these instructions, they're saying to go 0-20 degrees forward for a 5-point (single sub) and 0-20-percent back for a 6-point. That's mostly a junk-preservation thing on the 5-point, IIRC. Go with a six-point and try to get the 20 degree angle back from a vertical downward line from the camlock.