What are you calibrating and why? Do you have a quality multimeter to provide a reference, or can you find someone who has a good one to check the calibration of yours against? A cheap meter can be nearly as consistent as a high end meter, but the initial calibration cannot be as implicitly trusted as something like a good Fluke. You have to do the verification yourself if you want really trustworthy numbers, and it may be more prone to drifting in calibration, but it can work just fine.
The measurement at the battery terminals is a closer to correct value for the actual battery condition. The measurement at the inverter display is likely to suffer from voltage drop through the wires, and will vary in its readout with changing load on the inverter output. The only way to reduce this readout discrepancy is to increase the size of the wires running to the inverter.