Multi-line range used for summing specific column

Thought I'd give a sense of what I'm dealing with, as I'd love thoughts on best way to implement pivots for this.  The attached is a fraction of an extract, which shows the basic dynamics:

- there is a section of assumptions (this is one line of assumption...there are about 20-30 per section, and currently there are 20 sections)

- there is an actual time of months (this shows 12, but it's a 3 year model, so there are 36 columns of months

- this is a set of assumptions that occur on an absolute time basis (the assumptions say between months 1-5 there are 50 members acquired, and then from month 6 onward there will be 100 per month)

- there are certain assumptions (not shown here, to keep it less complicated), which are 'as of joining' dynamics...so it'll say from month 1-3 there will be no fee charged, and then from months 4-6 a 50% fee will be charged, and then followed by full fee).  This means that for a customer joining in month 3, their first month (the "month 1" as of joining) is in actuality month 3 of real time...so the 'as of joining' assumption would be applied starting in actual month 3, for 3 months (up to month 6), then the as of joining assumption between month 4-6 applied to actual month 7-9...

and the above dynamics would be recreated for every month's group of customers, for 36 months...and there will be between 20-40 versions of all that.

Hence I put it horizontally for time, sum it up by section vertically, and need to be able to expand/contract the spots vertically that I'm capturing.

Currently the file is 14K lines long, and that's just for the revenue assumptions of the various scenarios we're envisioning (this is a broad platform we're creating).

That said, I'd LOVE to do this with pivots.  The only way I could think of doing it would be to flip the entirety of my calculations into a vertical stream, where each line represents one month's worth of one customer's worth of data. 

If you have ideas of how to efficiently use pivots to calculate the elements once, and then be able to flip them around for various reporting needs, would welcome your suggestions.

Thanks.