Slitherine

We had a situation where I had a 3 base cavalry unit (it had lost a base earlier) in the following formation:

XX
X

The problem is that, when wheeling and moving the rear base ended up in an open field - leaving the BG disrupted.

If the base was shifted onto the other file it would have been out of the field.

Is there a rule for how you set your rear ranks because after combing through the book we couldn't find any?

We played it as disrupted as shifting the base to avoid the terrain sounded...well overly 'gamey' to use a nice word.

Feedback? Opinions?

First a couple of bonus answers:
only bases that are in terrain are dissordered.
dissordered and disrupted are not the same thing although they have similar effects.

Now the answer you have asked for:
you can only change the 'formation' of a BG by turning, contracting or expanding. Turns will result in a new formation facing to a side and the rear rank is not restricted in where it is placed. Contracting should be obvious as is expanding. You certainly cannot shift a rear base as part of an advance just because it suits.

So how does the "only that base is disordered" work?

If I have to lose 1 per 3 dice for being disordered, but only 1 base is disordered, do I lose a dice still?

3 bases would have to be in the disordering terrain for you to lose a dice.

Disorder works on a base by base case, so if one base is disordered as it only accounts for one dice then there is no effect as are not three or more dice.
So two bases disordered accounts for two dice in melee no effect for being disordered. However, in the inpact phase two bases add in four dice (two dice each) because when bases are disordered ordered you lose 1 per 3 dice the bases in the impact phase will only get three dice not four.

Also works for friendly cav and elephants, only your base next to the nellies is disordered so no dice loss for you, the enemy loses dice for being disordered if they charge the nellies as they are all within disordering range since they are in frontal contact and in range when in an overlap position.
It's a neat way the system works that you can be in a slightly bad position but not suffer ill effects for it in combat.