FWIW, SBR of five is theoretically the lowest possible, since a perfectly lit subject with shadow reading = highlight reading will still have local contrast variations of about 5 stops.

Here is the bonehead method of taking a photo under flat lighting with an incident meter when you are targeting the neg for alt process work or Azo. Say you have HP5, which nominally has a rating of 400. Go ahead and rate it at 400 since you are going to be developing the heck out of it. Double the 400 EI to 800 and plug that in your incident meter. Point the meter at the sky and get your exposure reading. Expose the photo and then develop it to the highest CI you can muster. Pyrocat HD 1:1:50 for 16 minutes will probably get you there. This will give a you negative that reasonably represents the scene in front of you in a way that will print well on pt/pd or azo.