The graph at the bottom of this article may be interesting this context, it's from 2014, but it's all I could find with a quick search: https://www.epi.org/publication/waiting-for-change-tipped-mi...
The tipped minimum wage hasn't changed since 1991. At around 88-89, the difference (in 2014 dollars) between tipped minimum wage and the minimum wage was 40% in 2014 it was 71%.
That depends on the state. For example, all west coast states have the same minimum wage regardless of tipped or non-tipped. See https://en.wikipedia.org/wiki/Tipped_wage
> In the state of Alaska, California, Minnesota, Montana, Nevada, Oregon, Washington, same minimum wage are applied for both tipped and non-tipped employees. Tips collected by employees in these states will not offset employer's obligation to pay the wage, and tips is the additional income beyond the wage paid by employer.
20% is an easy mental calculation, so that's what I do most often these days. I round up to the next dollar or sometimes next multiple of 5 depending on the total amount of the check.