Moen model 82000/2300 shower faucet reassembled but now shuts off at 12. How do I fix it? Where should the cartridge stem flat side be at 3/9 or 12/6?

Flat sides are at 12/6. and it sounds like you have the handle on upside down or the cartridge is in upside down. There is a c and an h on the cartridge to show c on the right and h on the left. One flat side of the stem has an extra groove in it. That should be on the bottom when off. Handle points down when off. Good luck