The CR default for LDO is 3300 (mV). The LDO obviously will not be that value exactly, adjust it to what yours reads (GND, "3v") after the CR was powered on for a minute or so.
Calibrating gain: you could simply apply any load, you can do this in any range, I suggest the mA range (least noise sensitive).
Then measure the raw output with the DMM, adjust gain until the OLED reading matches.