KL43Z Load Cell with INA 125P
I don't know why your measurements are so unreliable and/or wrong.  You might have a wiring problem or a software problem.  I'm not going to try to resolve that.

I'm going to address the gain set resistor.

I looked up how the mV/V rating works (the Wikipedia site on load cells explains it clearly enough.)

You are powering the load cell from 5V.  So, the 1mV/V rating means that you will get 5mV out of the load cell  when you place 20kg on it.

The ADC on your development board seems to be a 12bit ADC with a 1.2V reference voltage.

You will need to amplify the 5mV from the load cell to 1.2V in order to get good resolution.

I come up with a gain of 240.  Now, you can choose the gain resistor.

Refer to the datasheet for the INA125. There's a description in there for calculating the gain for a given resistor value:  G= 4 + (60k/Rg)

You can rearrange that to go from gain to the needed resistor:  Rg = 60k/(G-4)

So, you need a gain resistor of 254 ohms.  The next closest value in 1% tolerance parts would be 255 ohms.



There's probably more to it than that.  The datasheet for the load cell mentions an offset that you'll have to account for.  The mV/V rating is also not perfect - it can be higher or lower by a bit.

You may also not want to go full scale on the ADC - you need a little "headroom" to cover inaccuracies and whatever happens if you put too much of a load on the cell.

This was intended to give you some idea of the process beyond "should I use this value that I don't understand, or this other value that someone else used that I don't understand, either."