I have the device hooked up to the computer to read the serial monitor while slowly removing water to see the moment when the moving average sinks below the alarm weight.

The reservoir holds exactly 400mL of water. I have the device set to alarm at only 15% water remaining, which would be 60mL (conveniently about 1/4 cup). By slowly removing water up until the alarm point is reached, it should alarm with 60mL of water remaining.

This snapshot shows one a cycle in which the moving average falls below the alarm weight, and as intended this triggers the alarm.

Success! The scaling and alarm calculation works, and the device alarms as intended. I'll be doing a test run tonight actually being hooked up to it. As seen below, I have a loose fitting bracket on the tub to relieve strain (and to keep it from drooping down by the radiator). 

One issue that could arise is that during night movement, I could pull on the tube, changing the amount of strain, and therefore weight exerted by that strain. If the tube is pulled tight, it could reduce the measured weight. If the tube is made looser hanging, it could increase the measured weight.

I will certainly need to improve this so that the tube is held tightly in position at the bracket, so that it has little-to-no effect on any measured weights. It will be interested to do a few tests and see how much this can affect the results.