Tiff - Great question, it almost always comes up during the Annual when the org check up is reviewed.... "how do we come up with a number for everybody?"
The solve I have seen a bunch of clients implement is taking this issue to the next 555 for folks who do not have the Number. Spend 5 minutes on Core Values then Spend 5 minutes on Roles and ask:
"Hey, what do you think is a good weekly thing you can measure that will make you know you had a great week?"
The key is to Flip the Script, to make this Number something that is seen as being doing For them and not To them... EOS can often be seen by employees as an onerous thing being done To them. Flip the script and get them to come up with the Number.
The 555 is the perfect time because it is a meeting where we have two lives working on one, the 555 is For the employee.
Hope this helps.