Waldog, let me caution you:

1. There's no evidence that borax will help much with mustard algae. It won't hurt, but it may not help either.

2. You are wasting time and money messing with phosphates without testing. You could lower your PO4 level from 4,000 ppb to 400 ppb . . . and accomplish NOTHING. You have to get it below 125 ppb (0.125 ppm) to accomplish anything useful.

If you really want to try it, search online for a quart of Orenda PR-10000 ($50 - 60 delivered) It's very unlikely that your pool has more that your pool has more than 10 ppm PO4, and that product will remove up to 20 ppm from YOUR pool. Clean your filter and THEN add no more than 1/2 cup. Wait at least 4 days, before cleaning your filter. Repeat this 5 more times (clean filter, add 1/2 cup, circulate for 3 days). Most likely that will work.

But you'd still be better off with a test kit.