It depends on the number of units in the hero's army. If the hero's army has only one unit, it will always be in the middle. If there are 7 units, it will end up where you place it.
To have a unit at the very bottom, you need to have, like, 4 or 5 units... I don't remember exactly...

In general, experiment with the number of units in the hero's army.

Well, that's what I did.

Here's how it works:

--with a loose formation (top to bottom, from first to seventh):

An army of 1 unit starts on horizontal line 06.
An army of 2 units starts on horizontal lines 03, 09.
An army of 3 units starts on horizontal lines 03, 06, 09.
An army of 4 units starts on horizontal lines 01, 05, 07, 11.
An army of 5 units starts on horizontal lines 01, 03, 06, 09, 11.
An army of 6 units starts on horizontal lines 01, 03, 05, 07, 09, 11.
An army of 7 units starts on horizontal lines 01, 03, 05, 06, 07, 09, 11.

--with a tight formation (top to bottom, from first to seventh):

An army of 1 unit starts on horizontal line 06.
An army of 2 units starts on horizontal lines 05, 07.
An army of 3 units starts on horizontal lines 05, 06, 07.
An army of 4 units starts on horizontal lines 03, 05, 07, 09.
An army of 5 units starts on horizontal lines 03, 05, 06, 07, 09.
An army of 6 units starts on horizontal lines 01, 03, 05, 07, 09, 11.
An army of 7 units starts on horizontal lines 01, 03, 05, 06, 07, 09, 11.

I hope this information helps someone else.

P.S.: If the position is so important, then the Tactics skill exists for that purpose.