20 ppm is fine, if you have a concrete pool.

If you have a vinyl pool, I'd recommend trying 6 gallons at a time first, given that your stabilizer level is so low.

Also, brush the pool after adding the chlorine, and then use an OTO / phenol red kit to check your chlorine, in the AM. If it's below 5 ppm, add more.

IMPORTANT: when the chlorine is above 10 ppm, you can NOT test with a K2005: not chlorine, and not pH! With the K2006 or the K1515, you can test chlorine to 50 ppm accurately, but pH testing is still limited to 10 ppm. There's a work around, but you should not need it yet.

The cheap OTO kit can't test pH above 5 ppm chlorine, but the OTO will go to 50 ppm or more. It's not very precise, but it's very dependable.

With your low stabilizer, the algae killing range is dark yellow to orangish-yellow (~10 - 20 ppm)