the gap length you arrive at is different from the one
in the NEC table
[for different sector sizes]
Part of the calculation is the time it takes the data separator to re-sync
after each gap. Now, a "true" 765 doesn't have a data separator so maybe
they leave this out. It's also true that this number ought to be nearly
constant depending on sector sizes.
Do you have the Intel 8272 data sheet? (This was Intel's part number
for their 765 clone. It may not just be a clone but the exact same
mask...). IIRC there are more details/examples/math in the 8272 data
sheet, including some recommendations for data separators and the good/bad
of different types.
Tim.