You should be able to build an ESI calc field pulling from Business Process Event Records Completed, using a T/F to establish it is the step you're looking for (Step in the selection list [relevant step]). If you use Completed date as the sort field and sort descending, first instance, it will pick up the one that was most recently completed.

It sounds like you probably already have an ESI field, so you could copy that and change the sort direction or, if this is the only place it's being used, you could change the existing field (but I stress, only if you are sure this is the only place it's being used / that this is desired behaviour for other usage too)

Apologies if any field names are not 100%, I am on mobile.