I like a chair height of 18", so with a 3" cushion you would build the bench at 15". Your cushion might only be 2.5" thick, so you could allow for that by making the bench 15.5" high. To determine what height you happen to like take your tape meaure around your house to dining room chairs and breakfast chairs. I wouldn't measure lounge type chairs as they will usually have more give in them and thus are not comparables.
Carolyn Albert-Kincl, ASID

Thank you! This is very helpful!