Getting a feel for aperture increase?

I've been doing some calculation. It appears that for planetary
resolution and the Moon (not DSOs) and ignoring the difference in
contrast and brightness, going from an unobstructed aperture to
another is **approximately** as follows:

70mm == 4" is very close to going from a 4" to 7" or 8".
85mm == 4" is very close to going from a 4" to 5".
From my calculation, it does not appear that there is much difference
in the resolution going from the 7" and 8" aperture.

How does this model correspond to your actual observing experience?

Thanks,
Ron B[ee]