Loadsheets Calculations
If you are doing weight and balance calculations mathematically and using the correct bay centrums from the applicable manual for that aircraft either pax or cargo, then your end result should be very accurate.
If you're doing a manual loadsheet using the same bay weights and interpolating the index from a list of weights, you will always find a slight difference, additionally it will depend on the accuracy of the ZFW/TOW graph and the thickness of the pencil you use. Either way, the mathematically produced loadsheet will always be more accurate. It's the final %MAC you need to be looking at, a difference of more than 2% at TOW should be considered too much.