Do you use any (process) maturity models (e.g. PMF, CMMI, etc.) to calculate scores?

No, we do not.

(Process) maturity models base their evaluation on questionnaires that need to be filled by people. The issue is that the quality of answers is dependent on the knowledge and experience of the respondent and may be biased. 

We have addressed this issue by analyzing data in our clients’ ITSM systems, giving us an objective view on the level of execution of the analyzed process.