I've thought about this. My "15%" standard in application is more like. 10% + 50% of 10%, ambiguously round to whole dollar. So if the bill was $45, I think, $4.5 + about $2 is $6.5 so I'll give $6 but maybe I go $7. I never do cents unless it's to force the total to a whole dollar amount (rare).

I'm moving towards a fixed amount. Since, as I mentioned elsewhere I don't see a strict correlation between menu price and work performed. The amount of work to bring me a steak is same as work to bring me a burger; but the steak might cost 3x more. Majority of dining experiences follow the same script and same amount of interaction with waitstaff so I'm thinking of just giving everyone $X and don't even consider what I spent. Maybe adjust up if we had appetizers, extra beverages, or some difficult situation. Having a toddler, I've left my fair share of huge "sorry for the mess" tips and that doesn't bother me at all.