The most common rule-of-thumb is that proposed by John Allen. For HO, this is expressed by the formula 32/R, "R" being the radius in inches. The result is the additional grade percentage to be added to the nominal grade. For 22" radius, the result would be 1.45%, added to your 2% nominal grade would be 3.45%.
Empirical testing is underway on this calculation and it seems to be relatively accurate. The table posted earlier doesn't seem to relate to observed curve effects on grades.