I'm not sure if I can explain this point clearly without a diagram, but I will try my best.
I am looking at the situation where a deployed battery limbers up and moves away from the direction in which it was previously facing.
The rules seem to say that I can place first limbered marker with both of its front two corners registering with the front two corners of one of the deployed bases. I can then pivot around one of those two corners to set my initial facing.
If I want to move to the rear then I would pivot the limber base 180 degrees. So now the base is sticking out forward of the former position of the battery by the depth of the limber base (120mm in my case with 15mm figures). Then I would measure my move from the front of the limber. So effectively my move is measured from the line made by the front of the battery where it was when deployed.
The above is what I have been doing in my games.
Now the rules also say that I cannot form up forward of the chosen base. So presumably if I do what I have described above I have broken that rule because the whole of my limber base is forward of where the unlimbered bases were.
So taking that into account it would seem I cannot pivot on a forward corner of an unlimbered base at all because as soon as I do I will have moved forward of the previous line.
Does that mean that I have to limber facing in exactly the same direction as the unlimbered bases and then use my movement allowance to wheel to the rear and then move, thus losing a good chunk of the move wheeling?
Also with either of the above options, if there is enemy within 120mm of the unlimbered battery, neither methods of moving would be possible as that falls foul of the interpenetration prohibition?