Consistency formula

Hi there,

Please excuse my mathematical ignorance here, but I'm looking for a formula to calculate consistency of attendance of individual people across a number of events.

To expand - an event, and 8 people (the static figure) are to show up, there is a consistency of 100% because there is only one event, with two events, if we keep the static figure of 8 people, and we have the original 8 for the first event, and four of them return for event #2 with 4 new people, we then have a 50% consistency, and so on.

I've calculated it given a static number of attendees per event, but when I change it to be more dynamic and then need a function

The formula that I have for the static scenario is as follows:-

(((Events x StaticAttendance) - DifferentAttendees)/StaticAttendance))/((Events-1) x 100)

The above formula works given a static level of attendance. However, for obvious reasons this doesnt work if there are 8 Attendees in event #1 and 12 attendees in event #2.

If we had 12 different attendees across the 2 events the consistency would obviously be 100%, but even by trying to reverse engineer or amend the above formula by factoring in differing attendee totals I can't crack it.

I know this is a long-winded explanation to probably a very simple algorithm, I'm struggling and would really appreciate any help someone could give me.