This post moved to this thread from another by request. (Dont know how it ended up on top like it did).
Pogueld is the OP of this thread.
Evaporation and refill will only add CH from the fill water. It will not lower the CH. If your CH is getting lowered, then that can only happen with water replacement (or precipitation of calcium carbonate as with scaling). Perhaps you have a water leak (see Leak Detection)? Or do you have summer rain overflow? According to this link, you had 8.2" of rain in May and 4.8" in June. Unless this rain all came in a small number of days, it usually wouldn't overflow and instead it would just raise your water level and then evaporation would drop it down.
Perhaps your brother-in-law can explain why he sells calcium at all. If he says it is to protect plaster surfaces from dissolving their calcium carbonate, then just say that's why you are maintaining the CH level. If he wonders why the CH level is dropping, then tell him the reason (i.e. you have a leak or you've had summer rain overflow diluting the pool water). It is NOT from evaporation and refill as that can only increase CH, not decrease it. Evaporation only removes water (volatile chemicals can also evaporate, but that's not anything in the pool with calcium).