The HX711 and loadcell don't necessarily return a linear a value. It is more likely to be a curve.. the nature of materials. You can create a variable and factor all your read values by this amount. For example in your case 500/268 = 1.87... So assign this value to your variable and multiply you readings by this amount. This will be approximate and it will vary for different weight ranges but you can keep refining it until you are happy. You can extend this method over various weight ranges and write conditional statements to apply different corrections for different ranges. Or if you have the smarts you can write a formula for the curve and apply it to your read values.