You've already added 2 boxes, so deduct those, but yes, 7 boxes was what I intended. Bleachcalc has an error in the borate calculator. Feel free to work up to 60 ppm in 2 box doses, if you prefer.

FC = 20 won't necessarily kill it, but it will likely stop it. Mustard algae doesn't take over the pool suddenly, so if you get to 20 ppm, and brush, and still see it recurring -- add more.

When testing, use the 10ml sample size, rather than 25ml sample, At 10 ml, 20 ppm = 40 drops, not 100.

A cartridge filter will normally trap dead - but not live -- algae.