Squish Band Width Calculation on Head Motor 2 stroke

examples of book 2 Stroke Performance Tuning - A Graham Bell

In the example in his book Graham Bell, the size of the squish / 54 mm piston squish band means it is 8 mm or 50% referred to the Squish Band .....


whereas from book 2 Stroke Tuner's Handbook by Gordon Jennings

Graham Bell is the same as 50% Squish Band ...
question:
If the piston 54 mm then 50% of its squish band is 8 mm, where the 8 mm it? How is the formula?
Possible answers:
If the second reading of the book, NOT FOUND Formulas for counting the squish band .......
Nevertheless, the following formula would be used as a reference (at least by me yes, hehehe)
So the formula:
r = 0.5 * D * (1 - sqrt (1 - S))
it is the root sqrt ...... :)
where:
r = width of the ring --------------------> squish his band
D = cylinder bore / piston size
S = squish factor as a decimal fraction, ie 0.73 not 73% (factor squish his band in decimal)
example:
for example, in Graham Bell mentioned a 50% squish band (S) of the piston 54 mm (D) that is 8 mm
so S = 0.5, D = 54 mm, then
r = 0.5 * 54 * (1 -sqrt (0.5)) = 7.9 mm 8 mm rounded (very close to)
Meanwhile, Eric Gorr in his book - 2 stroke top end and performance
Head squish band with a width of 60% of the bore / piston and compression ratio 9: 1 suitable for an emphasis on the lower to middle RPM ...........
Head with suish band (35-45%) of the bore / piston and compression ratio 8: 1 suitable for an emphasis on upper RPM.