Rear seat mount dimensions?

I'm making a platform to replace my rear 40% split to house a fridge.

I'll be using all four of the existing Toyota seat brackets to mount it.

I've already measured that the rear holes are ~18" between centers, and the front flip mounts are ~8.82", and they're ~18.5" apart (front to rear), but what's the offset?

Are the flip forward mounts centered between the rear? I can try to measure, but it'll be very difficult to get correct.