I placed the stored calibration in a string in the same storage mechanism used for all the options you can set (e.g. the same place as the binary flag that says to reuse the calibration).
Its key is SerializedCalibration: invent.kde.org/education/kstars/-/blob/m...rs/kstars.kcfg#L2515
Honestly, I don't know where all those options wind up being stored. Perhaps one of the other developers can comment.