Access to logic behind standard Sales KPI?

I had a request to set up alternate versions of the Sales KPI to allow management to view results over multiple timeframes concurrently on their Dashboards.
I found the article in SuiteAnswers with suggested logic and created a custom Sales KPI.  The article contained a vague caveat about variances due to customer specific accounting differences and sure enough, my custom KPI doesn't match results from the standard one.

I've been trying to reconcile the differences between my Sales KPI totals and the standard KPIs by comparing transaction-level detail between the two but not making much headway.

It seems the only way to access detail for the standard Sales KPI is to modify the Sales By Customer report that it generates, which is really a different method of pulling data data than saving a search against Transactions?