What specific US Regulations dictate that decaffeinated means 97.5% reduction in caffeine?

The 97.5% reduction rule appears to be a coffee related rule.

Under a U.S.D.A. administrative regulation entitled "FSC 8955 A-A-20213B, COMMERCIAL ITEM DESCRIPTION: COFFEE (17-AUG-2004)," decaffeinated coffee shall not exceed 0.10 percent caffeine on a dry basis in the package. This appears to be the only actual regulation dictating the standards for decaffeination in coffee.

It is possible that the 97.5% rule derives from the incidental difference between the average amount of caffeine in a regular cup of coffee as compared to a decaf cup of coffee, as opposed to an actual, formal regulation.

According to the U.S.D.A. National Nutrient Database for Standard Reference (NDB), as outlined in "Basic Report: 14209, Coffee, brewed from grounds, prepared with tap water," the average cup of regular coffee contains 40mg caffeine per 100g. In "Basic Report: 14201, Coffee, brewed from grounds, prepared with tap water, decaffeinated," the average cup of decaf coffee contains 1mg caffeine per 100g.

Assuming the decaf coffees used by the NDB are compliant with the U.S.D.A.'s regulation on the maximum amount of caffeine permissible in decaf coffees, then, incidentally, the difference in average caffeine content between a regular cup of coffee and a decaf one is exactly a 97.5% reduction. The math: (40mg * (1 - 0.975)) = 1mg.

But compare the average caffeine content in teas (according to the NDB); 20mg caffeine per 100g in regular tea, to 1mg caffeine per 100g in decaf tea. There, only a 95% reduction in caffeine.