If you build a serial conection of resistors ,a maximum resistance will allways sum of any(first Kirchoff Law).

So you should add one another relay on in of A/C to set 0V (put to ground) before you change range.
Next you should change range and open relay on in of A/C.

You should use calibrated, temerature compensted resitors (chronicielnin or even oven). Second method is add a bridged mesurement.
There is much of specialized relays as Motorola and so on.