How does one calculate time differences per mile using PHRF?

If a C&C 99 has a PHRF of 99 and is only slightly faster than a 35s5 with a
PHRF of 132, wouldn't the 35s5 then be almost indistinguishable from an
Express 30 with a PHRF of 139?

Is it some type of square or cubic law?

Thanks!

Amen!