Hi, and welcome to the forum!!
Your CYA is not going to drop from evaporation in the water, but if you drain some water before refilling, you might lower it some. You do need to get some chlorine in the pool quickly though--it's amazing how quickly a perfect pool can turn green without enough chlorine. With your CYA at 100, a chlorine level of 3-4 is not going to be enough--for that CYA level, you need to maintain a MINIMUM of 8 ppm at all times, max 15 ppm. You can check out the "best guess chlorine chart" link in my sig for chlorine levels that correspond with CYA levels.
How high is the TA in your fill water? If it's higher than the 110, once you refill, you may have to repeat some of your TA work to get it back down. Part of the reason that you had trouble keeping the pH up before was probably the trichlor feeder--the trichlor is very acidic and will drive your pH down. Now that you've removed it, it makes sense that your pH is staying a bit higher.
Janet
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