Get a calibration you can trust
A calibration can finish without an error and still be poor. These habits make a good one more likely.
Move the board around a lot
Don't hold the board still or sweep it in one line. Show it near and far, tilted and turned, all through the space where you'll record. The more variety the cameras see, the better the calibration.
Check the calibration error
Each calibration saves its error in pixels. Lower is better. To find it, open the calibration file
({recording name}_camera_calibration.toml in the calibration recording's folder) in a text
editor and look for reprojection_error_px. If it's high compared with your earlier calibrations,
record a new one with more varied board views. See why calibration matters
for what the number means.
Recalibrate after any change
A calibration only fits the exact setup it was recorded with. Calibrate again whenever a camera is moved, bumped, refocused, added, or removed. FreeMoCap can't tell that anything changed, and keeps using your last calibration.