I'd like to know how low a temperature one should expect of a properly operating forced air residential A/C system measured at an air register (relatively close to the blower/evaporator unit) after the temp has stabilized at it's lowest point (10 min in this case). Also, I understand 20 deg F is a "nominal" target temperature differential, outside vs. inside temp. Where is this to be measured? At an air register, or at the thermostat? I'm getting 59 deg at a register with outside temp of 90. Thermostat is set at 79.

Thanks,
Eric

Now measure your temperature at the return air (where your filter is) Subtract that temp from the supply air temp.( your register 59 degrees)