Generally for every 10 ppm of FC added by trichlor you are also adding 6 ppm of CYA. Having said that, I think you better wait until you have finished doing your partial drains and refills before deciding to use trichlor tabs. Unless you get it really low, you're not going to want to use them. It's a long way from 300+ to low enough to use trichlor. My guess is that you'll do better using some form of unstabilized chlorine. Depending on what your calcium hardness reading is, cal-hypo could be an option for you as well as bleach or liquid chlorine of course. Hope this helps.