I think you need to think about it in different terms.
Under the worst case senario, the tractor is upside down and you are seat belted to the seat hanging there. The ROPS will be below your head and the front of the hood of the tractor will be the other point of contact with the ground. Depending on how your tractor is made, my guess is that in a roll over, the front of the hood will crush down to the radiator? Or some other semi-strong part as the sheetmetal (or plastic) of the hood is not going to support the weight.
SO, if it was me, I would flip open the front hood of my tractor and I would find the strongest thing under there that is likely to take the weight of the tractor in a roll over. I would assume that is the front support point. Then I would draw a line (run a string?) from that point to some point behind my head, but that still allowed clearance over my head of at least 6". That point, which is somewhere in the air both above & behind my head, is the lowest point that I would feel the ROPS needs to be.
I'm not an engineer, but make sure you have somewhere to mount that ROPS to. It will have to be a very strong point, not just bolt it to the fenders.