Best practice for creating Marketing List in Dynamics of Marketo leads

I want create a marketing list in Dynamics of leads that have completed a Marketo form.  This will need to be a scalable solution so I hesitate creating a custom field for this specific campaign.  I could create a dynamic marketing list in CRM pulling synced leads based on a specific value (aquisition program or UTM parameter), however if these fields change then the marketing list will break.

What is the best practice to do this?

Thanks

Jennifer