You can swim up to shock level. The primary danger to a pool from high chlorine is bleaching but that only really affects vinyl liners (your pool is plaster) and it only occurs at levels far in excess of shock levels.
I don't have the exact values, but I saw a thread where the poster accidentally went up to 3X shock level (because the poster tried to start the SLAM without a test kit) and they experience some liner color fading. But that was a vinyl liner pool.
You should be perfectly safe until the refills arrive. If you're concerned, then only use 80 or 90% of the three-day average and you'll be fine.
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Ok, thanks for the clarification. I think I'll "eyeball" it to FC20 since that's when I noticed the biggest improvement.