Hello Johannes,
Please refer to below link for possible Progress Measurement Methods:
Define Measurement Methods - Project System - SAP Library
What you did mentioned looks similar to Cost Proportionality and Quantity Proportionality. Besides you can simply use Estimate, that is when the business user does personal estimation in % value. Sometime that can be most accurate, since not dependent on neither plan nor actual data.
Automatic is nice, but requires good quality of plan data, as well as proper accruals for actual data. Your actual cost in the system is normally either behind or ahead of real work progress.
Regards,
Paulo