To save the calibrated plaque click the Save File As button in the Calibration window.
Plaques should be saved in the Plaques folder in the Plaque Simulator Data folder. Plaque Simulator will automatically add the filename extension ".iplq6" for you.
If you saved your new plaque in the Plaque Simulator Data folder it will appear in the Plaque files menu after you update the menu or quit and relaunch Plaque Simulator. Whenever you need to use this plaque simply select it from the Plaque files menu.