WBS Subtotal Function?

Hi,

I'm creating a WBS that has 300 subtotal durations. In the example file in Cell K62, I have the following function:

=SUM(SUBTOTAL_01,SUBTOTAL_02,....SUBTOTAL_55)

I tried to enter, SUBTOTAL_56 through SUBTOTAL_300, but I got a "#NAME" result instead of the total of these 300 Subtotals.

Does Excel have a limit of number of subtotals permitted? More importantly, how can I get the Total for all 300 Subotals? Is there a better function than the one I'm using?

Thanks