Is there a rule of thumb when it comes to hitting a slot jackpot ($1200 or more) and tipping when they pay you?

I had some good luck twice in the past two weeks, and when getting a handpay I gave the two girls each a $20 tip on jackpots of $2409 & $1701

was that fair? too much? not enough? I even asked them what is one supposed to tip, and was told it was up to me.

appreciate to know

thanks
Mickey