Here's what I had in the wiki:
Mpguino calibration - EcoModder
EDIT: THIS IS WRONG, THE WIKI IS BEING UPDATED!!!
"A. navigate to the "VSS Pulses/Mile" screen (hint, left and right buttons will "tab" to the next field"). Take that value, divide it by the number of miles the guino said you traveled, then multiply by the number of miles on the odo. Adjust the value to this new value and hit OK."
In your example, lets say you had 8208 in the "VSS Pulses/Mile" screen.
the new "VSS Pulses/Mile" value would be (8208/20.14)*20.00 or 8151.
you want to increase "VSS Pulses/Mile" if the guino reading is high, so that it counts more pulses before registering a mile.