Maybe I'm late to the party, but here's my contribution: If you want to get that rear edge perfect, just take the spare tire cover off and trace it directly onto the mat. There are two clips on either side near where it hinges that just pop right off!
I actually traced the spare tire cover directly and then added 2 3/8" on each side. You won't have the 2 1/2" "tabs" on either side where it overlaps the seat, but you can either plan for it (use Cardinal's measurements and then just trace the spare tire cover on the back edge)
or just make sure the other mat for the back of the seats is wide enough to cover when they are flat - I don't think getting that little corner of the seats dirty with the seats up is a major concern...at that point whatever muddy object in the trunk is probably gunking up the whole back of the seat
I enlisted the assistance of my wife who has some sort of rotary cutter for her sewing projects...it's like a little razor-sharp pizza cutter. Made the cutting REALLY easy...especially the curved back edge - just held the spare tire cover down on the mat and used it as a gate to guide the rotary cutter.
Great idea, Cardinal - and a lot cheaper than the "professional" covers!