First, ensure that when you defined your pool you chose plaster for Pool Type. Next, check to see what your Target CH is set to - should be around 350. Regardless of whether you set it up with liquid chlorine or SWG, the recommended hardness levels are the same;
CALCIUM
ACCEPTABLE MINIMUM:
250-350
IDEAL:
350-550
ACCEPTABLE MAXIMUM:
550-650
So 350 is at the lower end of the Ideal range. If the pool is setup correctly in the TFP app, then it shouldn't be asking to remove any water. If everything is setup correctly, then something is wrong with the app install. I would try uninstalling and reinstalling.
Regardless - if your CH is 350 you don't need to replace any water. I didn't replace mine until I hit 800 several years ago, about two years after the pool was built. Since then, my pool has cured enough that it's remained fairly constant in the 350 range.