How can we effectively measure the maturity level of a Project Management Office (PMO)? Are there any survey questionnaires available where we can input our answers and receive an outcome with recommendations? If not, could you please provide some references that we can look into?

I’m looking for references for Project Stage/Phase Gate and cost estimate accuracy at each stage/phase, e.g.: 50% at Ideation, 25% at Business Case, 10% at Initiation and Planning. We have something in place, but I wanted to see if the market and Gartner can give us updated references/benchmark. I tried some researches but nothing concrete.