The zeners shouldn't clamp if you just replaced RN1 with the 1Meg packs.
For you, you will want to replace:
float adcVoltage = 10.0*adcVal/1024;
float adcVoltage = 55.0*adcVal/1024;
If you used my sketch, the two numbers should be the same in Cayenne and on the display, save a significant digit or two.
I will probably use my PLCduino to control a home brewery eventually, although, I like having distributed controls, so may just use ESPs.