The same setting that specifies that the text be left aligned can be changed to specify center justification instead. Is that what you want to see here.
If you enter layout mode and select the layout object (either text with merge field or actual field object), there's an option for center justifying the text in both the formatting tool bar and on the inspector's appearance tab.
Sorry, I should have been more specific. I want to keep the text left-justified, so that each line of text is aligned like it is in the screenshot, but have the whole block of text in the center of the field.
Which is now the calculation for that field ?
( because you could change it padding the result with some spaces if it is too short )
There is no calculation for that field currently.
I'm not sure how I would go about getting the length of the longest line in the field and changing the indent, could you expand on that?