If you tip based on the post-tax amount, youare doing your server a favor if you are using the 15% or 20% formula. The bigger the total, the bigger the tip. The amount of your tip has no bearing on the amount of income tax a tipped employee pays. Tipped employees tax is based on 8% of their total sales. Not on what you leave on the table. Tipped employees actually make out on this government formula, but the government figured getting guaranteed tax on 8% was better than being dependent on the honest reporting of tips by the employee. That didn't happen much.
Tip of the day: If you look for a second job, this is the way to go.