The circles need to be relatively constant. Your boat is twin engine? Just make a smooth circle in it's own boatlength using the engines in any calm area. Your circle needs to be continually increasing with no decreases in heading while calibrating. You also need to make sure that the compass is reading all 360° with no missing quadrants if you're having issues with calibration.
I'm wondering why you're calibrating as I assume it's been done before. Is your pilot heading correct? You need to know where the pilots fluxgate compass is. You haven't stored a tool bag or anything magnetic near it, have you? Just checking.