If you’re trying to come up with purely quantitative metrics, I have a suspicion you’ve been neglecting your own responsibilities, which are less quantifiable.
Like 1:1s. That’s an opportunity to check in well before any periodic adjustment cycle. Because it’s more than numbers.
If a manager needs a bunch of quantitative data at the start of a review cycle because they don’t know how their employees are doing, it’s the manager that needs an adjustment. Down. Or maybe out the door entirely.