Tip on the pre-discount total, not the happy-hour line. If a cocktail is $14 and half-off, the service is still a $14 cocktail. 20% of $14 is $2.80. 20% of $7 is a different, worse night for the person who made it.
If the check already includes a service charge, do not double it unless you mean to. If it does not, 20% of the pre-discount food-and-drink subtotal is the floor we use.
Hours and prices stay on Deals. This page is the tip.