The 2005 Carve 122 should be quite similar to your previous Carve 121.
Once again, you need to do some "testing" on your own to find out where
the "optimum" mast foot position is for you, your rigs, and your fins.
I have always suggested that if you do not know where the optimum mast
foot position is the easiest way to find out is to move the mast foot all the way
to the back of the slot (make sure you don't get it so far back that the T-nut
hangs over into the wider T-nut access).
Then sail your board.
If the board is tailwalking or you have other control issues, then obviously you have the mast foot too far back.
Move it forward about 2 cm at a time until you are just able to control the "issues".
This should make your board as "fast and free" as it's going to get.
If you change the fin, or the rig size, you will need to repeat the "testing" to find the oprimum for other configurations.
It will normally be around the same point on the mast slot (maybe 1-2 cm forward or back).
If you want a "starting point and don't wish to do your own testing, I's suggest starting with the twist plate washer right at the back of the wider access slot. That works pretty well for me at 75 Kg. with 6.5-8.5 m2 rigs and a 32 cm Lessacher Duo Weed fin or
the stock fin.
Hope this helps,