New pool owner, trying to get my chemicals balanced

[mitchryan912]

I went back and tracked the weather in the timeframe my FC levels were quite stable, and I think I can conclude 2 things:

1. FC stayed stable because there were a TON of cloudy & partly cloudy days here in the beginning of the month.
2. The chlorine I've been using since the 1st has been some bargain deal chlorine from Menards that was on sale for $1.99/gallon.

I'm guessing that the former helped considerably, but the latter... I think the 12.5% chlorine I've been using lately isn't actually the 12.5% that it's advertised as. I put in 1 gallon earlier today, and it only raised FC up from 2.0 to 4.5 ppm. I added another 2 qts about an hour ago, and will be testing it before I go to bed and then again in the AM, to see if there is anything funny going on.

EDIT: Just checked FC at night before bed, and it's up to 8.0 ppm, which is in line with what you'd expect with 10% chlorine (2.0 to 8.0 via 6 quarts.)