I usually just add an external mounted ratio adapter of the percentage required, like these:
http://www.partshp.com/speedometer_gears.htm
It "skips to the end", bypassing the math and any detail measurement errors regarding tire rolling radius and such.
When I switched out the "Slim Jim" for a TH400 in my '63 Grand Prix the adapter I needed was already laying out on the workbench.
K