I also dose after use. That way the bad guys get nuked, and the chlorine level is very low for the next use. If we go a couple days without using the spa, I add some in between.
For me, 2 tsps. brings up the chlorine to the suggested level.
I'll add more for a higher bather load, usually it's just the 2 of us.
I use a couple TBSP's to shock.