While Coyle requires pre-tax tip calculations, at least one MSC (TJN) clearly states in their guidelines that the tip should be calculated post-tax, and even stingy ACL covers post-tax tipping (albeit sometimes below 20 percent).
When dining with friends or family members I can't ever remember anyone calculating the tip pre-tax (or tipping 15%, for that matter).
I do agree that there is no need to factor in service charges, "kitchen appreciation fees," and other such junk when calculating the tip.
@Rho wrote:
In the USA tipping is 15% or 20% or more of the bill not including tax. A pet peeve of mine is restaurants that try to have customers leave a 20% tip including tax.