Hi Julian,
Glad to hear you figured some things out. As far as calibration, the part is trimmed for gain at each of the gain settings for the ADS1247. Most customers do not need to adjust for gain outside of calibrating the sensor, where by the gain correction can be added in software.
As far as offset calibration, you should always issue at least the SELFOCAL. SYSOCAL might be necessary if you have some external circuit that would create an additional offset. In this way the ADC can make a correction for what would be considered 0V input. Like the gain calibration mentioned above, you can correct for external offset in software. Just remember to remove the offset of the ADC itself first by using the SELFOCAL.
I would suggest you just issue the SELFOCAL after setting the desired level of gain and see if you get the results you would expect. This is generally all that is needed.
Best regards,
Bob B