Catch43,
I suppose a perfectly broken in mitt is like beauty, it is in the eye of the beholder. In my opinion, in the perfectly broken in mitt, the pinkie side of the mitt will just slightly overlap the thumb side. Whatever you do though, DO NOT put one single crease through the middle of the mitt. Doing so will eliminate the pocket which is what you don't want.
There are currently two main schools of thought concerning receiving mechanics; frame around the edges of the strike zone using a long mitt, the other being receive everything thumb down using a box shaped mitt. The image in my post above will result in the box shape I am referring to. The image below will result in a long mitt which is better for framing:
Once the two creases represented by the red lines are formed, work on bending (a gradual bend, not a sharp crease) the mitt along the blue line as shown in this image. It will help make the pocket longer and allow you to receive the ball in the pocket rather than in the webbing, off your thumb, or off your index finger.
Yes, you can break in a mitt to any extent, but I don't. I make the creases and leave the rest of the leather rather stiff, It will close easy around the ball, but you won't prematurely wear out the leather and the padding.
One other trick I have learned from old time players which in addition to helping you receive will also help in breaking in your mitt. A catcher's mitt has only three fingers. Put your last 3 fingers (pinkie, ring, and middle finger) into the last large pinkie finger in the mitt, put your pointing finger into the middle finger in the mitt. This leaves the index finger of the mitt empty. Two good reasons for this. It saves a lot of bruised fingers when receiving breaking pitches, pitches in the dirt and any other pitch that for what ever reason, you don't catch squarely in the pocket. The second, and more important reason is that it spreads your stronger fingers out away from your thumb more, making your grip (and squeezing ability) much stronger. That is especially helpful when breaking in a mitt. Try it, it may feel a little different until you get used to it, but it does work.
Catch43, I misread your question---sorry. Yes you can
rebreak-in a mitt. Basically, just go through the steps mentioned and place the creases where they need to be and your mitt will suck that pitch in!!