Are you familiar with the concept of DIFOT – delivery in full, on time?
Typically, at the start of a job, your planning will include an expected delivery date. You might make an explicit promise to your customer.
At the end of your job, you know your actual delivery date. Did you keep your promise?
Compare the two (promises kept / promises made) as a percentage for a period of time.
This is just a lag indicator, telling you if there is a problem. It doesn’t tell you where the problem is. Another part of your Lean implementation should give you the tools to discover that.
The “system” can be anything from whiteboards or spreadsheets, up to a full-blown ERP system.