The rule (pdf 4.D, p47) is
Quote10.1. To turn 90o, a front corner is turned so that the corner remains in the same place and
the front edge of the base is where the flank edge was. Bases are placed next to the
base that has turned until the depth of the UG has been matched or exceeded. If the
UG is not in a legal formation then add a file on either, or if required, both sides until
a legal formation has been achieved. Then place the remaining bases behind.
I think this means:
The original front corner base that turns will still be at the front corner and the former front edge of the UG will be the new flank edge (exception noted below).
The width will be the minimum that both
- equals or exceeds the original depth (in BW, not bases)
- allows a legal formation
You can't expand the frontage any more than the minimum to meet the above criteria.
If (and only if) the minimum frontage that equals or exceeds the original depth does
not allow a legal formation then the former front corner might not end up as the new front corner, as one or more bases could be added "in front of" the former front edge of the UG.
Is that correct?
Sounds correct to me.
Assuming 15mm scale.
So an infantry UG in 2 ranks (each 20mm depth) turns 90 degrees, the first file will be where the existing two ranks were. It then must expand (because it must legally be two files wide). It can expand on either the left or the right side (assuming nothing else prevents it).
A cavalry UG in 2 ranks (each 30mm depth) turns 90 degrees to its right. The first file will have its left hand edge where the front edge of the UG used to be. The second file must be place to the right of the first file so that the original depth is covered. It cannot expands further.
Richard