800gph overflow is good. And the 1200gph with headloss will be pretty close, but I think it will be over the 800gph mark. You will need to add a T line off the return line with a ball valve to fine tune the return water so you don't flood. Or look for a pump that gets as close to 800gph without going over, with head loss.
Example:
Mag 9.5
Pumps 800gph at a 4' head loss
Pumps 750gph at a 5' head loss
Thats like right on the mark.