in 17,500 gallons of water, 1.5 gallons of 12.5% chlorine will add just over 10 ppm. I would start with that tonight, give it time to circulate, then test for chlorine after sundown. Test again in the morning before the sun hits the pool, and if you've lost any chlorine overnight, you need to maintain that shock level until you're no longer losing chlorine during the night when the sun is off the pool.

Janet