Meghan,
Great way to measure effectiveness of your e-mails.
One way to get this data would be to create a new Score field called something like 'Email Score Prior to Engagement'. The way you'd use this field is that everytime you send a lead an e-mail, you would increment score in this field by +1. In your campaign setup, you'll also want to use the crietria 'Member of Program is ANY, Success = False'. Adding this constraint will ensure that you're only adding a score in this field if your lead HAS NOT attained success in any program, which is your definition of Engagement.
You can then view data in this field to gauge how many touches it takes to engage a lead, since Score in this case would reflect # of touches.
You could similarly have another field and an accompanying campaignt o determine how many touches it takes to convert a lead into a closed/won account.
Feel free to e-mail me at info@roitargets.com if there's anything else I could assist with.
Hope this helps.
Thanks!!