Backwashing cleans your filter and sand....that's all you need. You may backwash anytime you like but I always look for a pressure rise first.

That 12.5ppm worries me a bit. How do you know that? It sounds more like you are putting in 12.5% Cl which may or not relate to ppm. How big is this pool (gallons)? If it's 10k, I understand. If it's not, you need to recalculate.