Marketo says engagement score is made up of it's own algorithm, they say it's a mixture of opens/clicks and more, I assume documentation is limited about it as it's structure is part of their backend. For Engagement programs it takes 24 hours to populate, Email Sends should populate after run.
Hi Katharine Bierce - timely question for me - I'm leading a training session for my marketing team today around nurtures.
As for insight into how the score is calculated, I can't offer anything beyond what Frank mentioned. However, I use this ambiguity around the score as a segue into introducing marketers to other more relevant reports. The Engagement Stream Performance report in Analytics is a much better option to help evaluate your nurtures!