Adding CYA and recurring algae bloom

[Poconos]

The consistant 1/2" loss per day is high. I get that and more here in northeast PA but ONLY when weather conditions are just right. Low relative humidity, warm water, cool night, and a breeze. I think you have a small leak somewhere. Could be joints in the real old pluming but may be the liner or one of the penetrations or skimmer or stairs. In other words, anything. If you're familiar with the bucket test you can do that. I've done it with a roasting pan, maybe 5" deep or so, fill it so it just floats, loosely tie is in place to hold it in the center of the pool, then watch the water loss in it compared to the pool. By keeping it in the center of the pool you keep the pans water at the same temperature as the pool water and keeping it in the center of the pool and low in the water presents it with about the same wind conditions. If you do find you have a leak then post in the 'construction and repair' forum and we'll take it from there. As for the drop in CYA, yes, it is just a dilution calculation. If your CYA is 50 and you replace half the water, the CYA will then be 25. As for the algae questions, I'll let one of the others handle that.
Hope this helps.
Al
Edit: I just saw your thread about Fix-A-Leak. Since you already did the bucket test maybe you are just dealing with high evaporation. To get better measurement accuracy you can use a ruler and measure the water lever above some known point on a step. In calm water you should be able to read 1/16" with a little care.