Determining if Horizontal Layout Group has room for an element.

I am building a UI Panel that will have a variable number of buttons, with different length text. The buttons should fill the panel from left to right, top to bottom. I want the buttons width to correspond with the size of the text, so the button sizes will be different.

I have setup a panel that is a vertical layout group comprised of horizontal layout groups. This works fine in the editor, the buttons resize according to their text length. However, I need to build these panels dynamically. I have prefabs for the outer panel(Vertical Layout Group), the rows(Horizontal Layout Group) and the buttons. I can instantiate them and add them to their parent and everything looks fine.

The problem that I am facing is that I can not figure out how to tell when a row is full. If I instantiate all the necessary object from prefabs, I can't see the width of the buttons. Also, the preferred size of the row comes back 0 regardless of the number of buttons I add to the row.

When I am instantiating GUI prefabs, is there anyway to know the size of the element, and the amount of space in a horizontal layout group?

This is a very rough image of the panel concept.