Proper way to tip?

My husband and I went out to dinner tonight and our check was almost $90 before discounts. It was my birthday dinner, so the dessert was free and I had a $20 off coupon. After that, the check was about $45.

I know generally, you should tip about 20%, but is that 20% of the final check price? Or the total check price before discounts?

I did tip her $20, which is more than 20% of the check before discounts, but I'm just wondering which is the "correct" way.