Most people will put the BP about 1" above the FB. I use Handymath.com for figuring my half moons. It you want to even make it easier, use a trapezoid instead of a half moon
I just figure out where my firebox will be located via Frank's figures then go 1-1 1/2" above and measure ID. Deduct for drain if including and make an 1/8" narrower for every foot so it drops towards drain pipe.... If that makes any sense... I'm horrible at telling how to do much of anything.... I'm sure someone else on here can explain much better
So the baffle plate will slope downward, to carry grease to drain, as it travels away from the fire box? Won't that cause a bit of heat to build up at the fire box end of the baffle plate?