What about using log scale and then using a $ per BF log scale? Because small logs take more work per BF, it is not wise to use the log scale. Plus, Schribner and Doyle underestimates the yield for smaller logs (under 20"), which means less money for the actual BF sawn when you are custom sawing.
Charging by the BF actually produced means you get a good return for large logs, but the harder work for small logs gives you a smaller return for your effort. So, maybe have two rates--one that is a little higher for logs under 14" and one for logs over 14".
Now, because walnut is so valuable, does it make sense to charge a little bit more per BF than if you were sawing soft maple? It does not cost you more to saw different species, but why should the log owner make a lot of money while you are just sneaking by?
How about making a deal where every fourth log is yours and you set the fourth log aside and saw what you can get after the customer leaves? Or maybe every fourth piece of lumber is yours when you saw all the logs?
Or maybe just give an estimate for doing the entire job, get paid in cash, and then do the best you can? This is how many trades work in the U.S. today...a quote for the entire job.