Flash power to Fstop

Flash power has a calculation based on the formula of: F = (Gn/Ft) * Ia

F = f/stop
Gn = Guide Number of the Flash
Ft = distance in feet from the flash to the subject.
Ia = ISO adjustment based on iso selected (values are powers of the square root of 2)
ISO 100 = 1
ISO 200 = 1.4142135623730950488016887242097
ISO 400 = 2
ISO 800 = 2.8284271247461900976033774484194
ISO 1600 = 4

then chart out each power the flash can give out be dividing the previous f/stop by the square root of 2.

calculated out using

Gn = 131
Ft = 7
ISO = 200

(131 / 7) * 1.4142135623730950488016887242097 = F
F = 26.46599666726792162757446041021
closes fstop would be 22

Power fstop
Full = 26.46599666726792162757446041021
1/2 = 18.714285714285714285714285714286
1/4 = 13.232998333633960813787230205105
1/8 = 9.3571428571428571428571428571428
1/16 = 6.6164991668169804068936151025525
1/32 = 4.6785714285714285714285714285714
1/64 = 3.3082495834084902034468075512762
1/128 = 2.3392857142857142857142857142857
1/256 = 1.6541247917042451017234037756381

Here is a java applet I've programed to help with the rounding of the calculation. This java applet does round the numbers to the first decimal point. Also I've added in rounding for Whole Stops, 1/3rd Stops, and 1/2 Stops.