Sounds like you shouldn't be using a repeating field for this, but
Field + Field + field  + and so forth.... + field 
Will compute the desired result as long as you use your field name in place of "field" and spell out all the repetitions instead of "and so forth..."
I thought it'd be a bit tidier using a repeating field; I've not really used them before and thought this would be a good case to try it out.
Guess I'll go back to the trusted separate fields.
Thanks for your help.
I hope that doesn't mean 30 separate fields. That would be almost sinful. If so, I recommend that you create a related table to store that data. Repeating fields should really only be used for display purposes.
+1 for David's suggestion.
I strongly encourage you to go that way.
Separate fields is certainly the worst way.
Though I discourage the idea, the sum of repeated fields would be relatively easy to set up.
Field + Field + field  etc
But it would be quite helpful if you can provide a more meaningful description. The idea of the reps 16-30 representing some different attribute as compared to 1-15 seems odd. What is this really all about?
I appreciate its not the best way to go about it, having multiple separate fields, which is why I thought the repeating field would be a good idea in the first place.
It's for gathering the required quantities of different materials on each floor of a building.
I have an Estimate table that has a related table called Quantities. Displayed in a portal on the Estimate layout, the users can select a product in the first column of the Quantities portal, then, they want to be able to enter the quantity required on each floor (that's what I was planning on using the repeating field for, the different floors, Ground - 29th).
The reason for trying to sum the last half of the floors was to truncate a printed report; where the quantities for Ground to 15th floor would be displayed, and then a sum of the remaining floors, so I could fit them all on a page.
Is there a better way of trying to accomplish what I'm trying to do? I'm sure there probably is, but I got a bit stuck on how best to add a product, then add different quantities for different floors for that product, in the Estimate layout.
As others suggested, use a related table with one related record in place of each repetition. If each record has a product ID and a quantity, you will have a number of options available for summing groups of records where the product ID is the same.
Self Join Relationships
are all possible options to get the needed sub totals.