I had a very shallow tall closet before my remodel - I don't know the exact depth but it was approximately two cans deep :-). It backed up to a structural steel beam so it couldn't be any deeper. During my remodel, it became even shallower - it's now one can deep :-).

I found it to be extremely functional because it held quite a bit and nothing was lost at the back.

Is there some reason you are making it shallow? I extended my cabinets into my dining area as well but they are normal depth - e.g. lower cabinets are 24" and I have two tall cabinets of approximately 24" on either side.

In terms of spacing, my narrow pantry now has adjustable shelves so I can change the height of the shelves when I want to. My former shallow pantry had fixed shelves and I adjusted to those as well - I would sometimes stack cans on top of each other if I had the height on a particular shelf and needed to squeeze stuff in :-).

Here's a picture of the cabinets I added to my dining area. I use the tall cabinet to the right as a broom closet.

It is actually a wall of 2 closets that is creating a hall-space and another nook area. The width is set based on a structural column (and to keep hall width 48" and maximize nook area)

And since we have a small galley kitchen, I am thrilled with any additional closet space so not unhappy with the depth. I considered adjustable shelves but I think will make them fixed to keep cost down.