I am ALWAYS running into issues where my summary fields or counting fields are coming up with the total count of customers in the found set,
That's what summary fields are supposed to do if referenced from a layout based on the same table where the field is defined.
Try this approach and see if it works for you.
a) perform a find on the customers table for all customers with a related record for the specified product.
b) use go to related records with the "match found set" option to pull up all related product records for all customers found in step a.
c) pull this up on a summary report layout where summary fields and sub summary parts produced the desired statistics. Be sure to sort your records to be consistant with the sub summary parts specified for your report.