As Watermom indicated, you need to verify your CYA reading -- and make SURE it's really that high.

If it is, it's probably not practical to run chlorine high enough to compensate for a 300 ppm reading. However, if you can find some granular sodium bromide at a pool store -- products like "Yellow Treat" -- using small careful doses can provide an immediate work around for the high CYA. So, do this:

#1 - Add 3 gallons of plain 6% household bleach per 10,000 gallons of pool water to your pool, ASAP
#2 - Purchase a cheap OTO/phenol red test kit, and use it to maintain dark yellow / light orange chlorine levels (10 - 20 ppm)
#3 - Once your chlorine is that high, brush your pool repeatedly.
#4 - Meanwhile, purchase a K2006 and do a dilution test of your CYA level, mixing 3/4 cup tap water with 1/4 cup pool water, testing the mix for CYA and then multiplying the result by 4.
#5 - Also, look around and see if you can buy a sodium bromide product -- you may need to call a couple of pool stores, and it may be sold as a spa product. Once you find some, add 2 tablespoons per 10,000 gallons of water to start. MORE IS NOT BETTER! And MAINTAIN your chlorine levels!
#6 - Tell us what bromide product you got, and how many gallons you have, and I'll figure a more exact dose.

These instructions should enable you to work out a process that will work till you can replace some water.