These are percentages. Inflation is already taken into account by the price rise in the service being tipped.

Maybe you mean to say that the base hourly rate was not inflation adjusted?

How does inflation influence the tip percentage?

Unless you're referring to wage stagnation, inflation should raise the bill and for the same percentage the tip should be higher on higher checks.

the bill has most def. been raised. Every time i take my family out ( 2 adults + 2 kids ) we're pushing $100 these days.

I've also noticed, and I can't say why this is, that the service or quality hasn't improved. You see the server two or three times. They probably don't write anything down and at least one plate is wrong somehow. A drink is missed. The only time I think I see a server more often is if I get a beer. They love asking me if I want a refill on those.