Fast Corner estimation for older devices

I made some tests with Apex families. Using the fastest speed grade I get approximately a relation of 2.5 between Slow and Fast model.

That was measuring tPD with a simple test design, combining a handful of inputs pins through a couple of LEs, to an output pin.

Assuming the Flex families are not that different, it seems that I can safely consider a rule of thumb of dividing by 4.5 from the Slow Model results on the slowest speed grade. In others words, I should constrain minimum tPD to be 4.5 times the value I really need.

Thanks again.