I read this article on tipping from an ex-waitress. (I thought it was politically correct to refer to them as servers nowdays, but I guess it’s OK for an ex-waitress to call herself whatever she wants to.)
I have a simple rule on calculating the tip that I’ll share. Take the first digit (the 2 of $24.50) and double it, to get a $4 tip. This is 20% for a $20 bill, but only 15% for a $29.99 bill. So you can either still closely to it, and hope the over- and under- tips average out with repeated visits, or you can do some rounding ($29.99 is really $30, so tip $6.)
I agree with the ex-waitress that 20% has become the norm for tipping and it’s easier to calculate than the old 15% tip.
But the point she makes that I agree with most is that if you cannot afford to tip 20% or thereabouts, don’t go out to dinner! The tip is part of the expected cost of dining out, so don’t skimp on it claiming that somehow it breaks your bank. Go out less, or go to less expensive places.