You on track with the bleachcalc.

The test is the same as you have been running to determine the chlorine levels.

Two things can happen here
1) when you get very high chlorine the indicator dyes seem to bleach out and give you a false low reading. Watch it when you first mix it. The color will be very intense for a couple seconds and then fade. It happens pretty fast. Diluting with distilled water can help avoid that.

2) you max out the test and have no idea how much you have gone beyond the limit.

This is important to understand with Chlorine levels but critical on the PH test.
You could be beyond the range ( most go from 7 to 8 ) and be dangerously acidic or basic and only thing you are at 7 ( or 8 ) when in fact the water could be 6.1 and look like 7. So with this test if you get a 7 or 8 do not swim until you fix it and fix it soon because you could be damaging your pool gear.

and yeah ... that's how it sneaks back and why some folks can fight it for 2 weeks and others will blow it away in 24 hours.
Get the chlorine levels up to the right level and keep them there. Don't wait 12 hours overnight to retest to see if its back down. Nail it again in 2 hours. If it has gone down its killing algae. When it stays high, you've got enough in there and you've almost killed it all.
The second half is not just having there, but make sure every last bit gets exposed to the chlorine with brushing. Most pool store sell a small round brush, somewhat like the pot scrubbers you use in the kitchen. Get one and use it in the corners, folds in the liner, around the drain and skimmer etc.

Good luck. I got a feeling you're going to blow it out fast.