Hi John,
Sorry - there was a typo in my last comment .. its "Make to Order" and not "Make to Stock". Pls replace it.
What i mean is Forecast will never get consumed by SO in SPP.
Yes, SPP relies on Historical data and calculate forecast based on the histories.
You can create forecast manually for a product location in SPP.
In ECC- SOP you'll create PIR manually, in SPP/APO DP system does the same for you (just an automation).
Regards
Vinoth