How to convert A/D data to the needed representation
Two methods:

(1) theoretical - get the specs for your load cell, analogue hardware, A-D, etc, and do the math

(2) practical - calibrate your system by applying a series of known accurate loads, logging the A-D output and then derive a calibration offset and gain (assuming everything is supposed to be linear)

Ideally you should do both, i.e. do (1) first and then do (2) to calibrate out any offset or gain errors in your system.