FortiCloud Premium - Organizations for MSP/MSSP

We just got FortiCloud Premium license to manage our customer's FTNT cloud services like EMS Cloud under our master account/org. But I'm struggling to set those customers under "Organizations". 
Originally I thought we would have to create an Sub-OU per customer then put the customer "account" under it. However, I realized a Sub-OU can have multiple accounts under it. Also when I "Add Account", I have to put Company Name in addition to name and email address of the account.
So now I'm wondering if I should create an Sub-OU like "Customers", then I might need to create customer accounts like "Customer1", "Customer2" and so on.

I'd like to ask people who work for an MSP/MSSP how your company is managing your account under "Organizations".

Also I'm wondering if the email address and the name I fill when I add an acount to suborg automatically becomes "Master User" in the diagram of this documentation page.
https://docs.fortinet.com/document/forticloud/25.4.0/organization-portal/999189/user-permissions

I asked this to an SE then even opened a CS ticket. But neither of them can give me the clear picture how this would/should be set up.

Thanks, 


Toshi