What is the lowest calibration point I can use when calibrating a load cell?
You can calibrate as low as you want if your load cell has the resolution to do this based on your criteria requirements.
For example, If you must comply with ASTM E4 standards, which says you cannot record data below 200 times the resolution and this value is above your lowest calibration point, it would not make sense to calibrate that low since you would not be allowed to record data at that level anyway.
If you know your resolution is better than 1/200 of your lowest calibration point you can calibrate down to that and still be within ASTM E4 requirements.
Regardless of your calibration range, you must adhere to the compliance requirements whether it is ASTM E4 or another standard.