Well, that's relatively easy to figure out, the test strips show you what you're at before adding bleach, say that reading is currently 0, if it takes 1 gallon to get to 5, then it will take 3 gallons to get to 15.
If you allready have a chlorine reading above 0, you can still figure it out with basic math. If you add chlorine and accidentally get 20ppm, it's also no big deal.
I didn't even bother adding chlorine to mine for as long as i've owned it (when closing), I clean it really good, drain below skimmer and return and walk away, don't bother covering it either. It's allready too cold for anything to grow in it, and all chlorine would be gone over the course of the winter anyways since I don't bother covering and the sun will eat it up, so in spring time it'll be 0 any way i look at it.
I open relatively early, let the vac run for a couple of days to get the dust that accumulated and we're ready to swim.
I really don't understand all the "you gotta do this and you HAVE to do that otherwise beware" attitudes, it's a hole with some water in it that all you have to do is keep clean, yet everyone thinks it's a high maintenance rocket ship!