We were facing the same dilemma. We usually use Programs, not Emails, within our EPs. When looking at the Engagement Stream Performance Report, every email had a score of 90, regardless of the responses (on average, our CTR is 4%). What's happening is Marketo is basing this score on the success within each program AS A PERCENTAGE OF INDIVIDUAL PROGRAM MEMBERSHIP (not EP membership). That's the key here. Even though you might have 300 members of an engagement program, Marketo is looking at program-level membership. So if you were like us, only those success statuses were being calculated in each program (there were no other types of members).
What you need to do is to define program membership for activity prior to success, e.g., "Delivered Email" (which is a program status we use and apply to all leads where an email has been delivered (we give this 24 hours to work)). With this approach, Marketo can then more accurately create an engagement score/ratio by dividing program success by total program members (again, not to be confused with EP members).
I think this would be a lot less confusing if Marketo would just base the score off of EP membership, and not indiviudal program membership.
BTW, I also learned from Support that "we only include leads in the Engagement Score who were members of the program or reached success during last 3 nurture cast (this is an internal calculation limitation)."
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