You can guesstimate an R-Value but in a situation like that it would not do much good. Example would be. in an attic Say you have 1000 s.f. of total area and 500 s.f. is insulated with R-38 and 500 s.f. has no insulation. The average Effective R-Value is not an R-19 or comparable to if the attic had an R-19 over the entire 1,000 s.f… It would probably be more like about an R-5 or something. Energy goes to the path of least resistance. There was a study that said something like an attic area that has a 5% void will loose 25% of its overall rated R-Value.
I used to be an insulation contractor and had all this in my head at one time.
One thing I do remember was that an R-38 Batt installed has an effective R-Value of about R-27 (because of gaps and framing members). Blown is much more effective!
Also an R-19 batt is about R-13 if the installer “takes great care” when installing. (a little known fact is that a standard R-19 batt needs a thickness of 6.25 inches and so if you squeeze it down to 5.5 inches (2x6 wall) it is R-18.)
If you really want to get technical and nerdy… you can do these calculation of overall efficiency using ResCheck. (free program, just google it) Which is actually what is or can be used to show energy compliance now instead of saying a certain R-value is required for in a wall or attic. There really isn’t a minimum R-value for a component (wall, ceiling, floor) any more in most States or Jurisdictions. And all houses will be different. even takes in orientation to the sun and windows as well as Climate.
Now how is that for a short answer