That's a really good question. I faced the similar issue recently. In our case, it's more complicated. We have 40+ brads with in the same instance.
If you are going to use the custom fields for the subscription management, you will not be able to see the unsub numbers in the email perforance report. This has to be captured separately. The approach we took was completely differnt and a bit more complx as we have larg number of brands. We created three custom fields, 1) Subscription Status 2) Brand Code and 3) Email ID. We add email ID as a query parameter strring in the subscription center URL and capture the same in the "Email ID" hidden field. Since we have separetesubscription center for each brand, the "Brand Code" is a static value for each page.
As soon as we get the updated preference in Marketo, we sync the same with SFDC and in SFDC we created a process builder to copy these update communication prefernces in a custom object. We synced this custom object in Marketo and use the same while defining the exclusion critera.
If you need to see the unsub report per email, you have to use the data of this custom object and then cobine it with your email performance data.
In our case, it will be easier in near future as we are creating a data warehouse to store all the data from Makreto and SFDC and will create these kind of report from there itself. Hope this helps.
Just came upon this discussion as I'm trying to solve for that exact issue in our email reporting, to easily track by email what the actual unsubscribe numbers are when we only use custom unsub fields. The solution to pass Email ID in as a query parameter thru the custom unsub form is great, but I got lost when you mentioned passing that info back into SFDC too and into a custom object. Can you explain why we would need that data in a custom object pls? If i just housed the field "Unsubscribed from Email ID" as a regular field on the Person/Contact object, could I not pull that into reports or smart lists the same way?
Thanks Grace, I will look at the documentation. Does this require having work spaces in place?