Hi Lauralee, and welcome to our forum.

I'm fond of Intex pools despite their shortcomings. My first was a 15' donut by them.

If it's too tough to try to measure in 3.5 cups, you can safely just figure a quart...with all your algae much will get eaten. In fact, if you just add 1 gallon, it should raise you to almost 14, which is still ok.

Without stabilizer, you need a chlorine level of at least 12, but over 15 may bleach the pool a bit.