What is the voltage of the 7808? You can program the gain of the INA128 so you don't go over 3.3V. The calculation is in the reference sheet for the INA128. If your full range is 16mv, then a gain of 206 should keep you under 3.3V, to be safe, i'd go down to 200. So from the sheet it actually tells you the resistor to use, which would be a resistor of 249ohm. If you use a 500ohm pot, then you can dial it in to what you need. Also, what is the quality of the load cell? Is it a $600 omega, or a $7 cheap ebay load cell? You don't want to attach S- to ground. Like I said before, I am not sure how well the 128 responds to single supply, since the info sheet does not even show an example. If you look at the 125P sheet, it will give you an example of single supply and what to expect.