I wonder how many people consider crank balance? Years ago I had my A10 crank reground at a local engine remanufacturing workshop, they asked me if I wanted it balancing. I said ok and took my rods and pistons in. They static balanced it and dynamic (end to end) too The result was a very smooth free revving engine When I ordered a crank for my B31 Phil pearson wanted to know the weight of my piston assembly so he could balance it, I weighed it to the nearest 0.1g on a digital scale. The result is an engine that is as smooth as Guinness and will happily cruise at 70 to 80MPH
Why don't you ask him? At the worst, we won't know any less than we do now.
I've tried 65 and 60% on my BSA singles race bikes, and both raise blisters at full song. Other road bikes I've rebuilt with Carillo rods and JE pistons and no re-balancing, and I can't tell the difference at legal road speeds.
I've never owned a single, but I can attest to the benefits of balancing a twin (A65), based on direct "before and after" experience with the same engine. In that regard, it has been posited that, more important than achieving a particular balance factor, is getting both sides the same, which doesn't apply to a single.
Did Phil say what balance factor he used? The manual says 58% on a DB34, if I remember correctly.
I have a new Phil Pearson DBD crank and piston on the shelf so it would be straightforward to determine the balance factor. However, I'm not volunteering to take the time to do that because I don't need to know the answer, and I strongly suspect the result would be 58% anyway.
I'll be going through the identical process shortly on my Ariel rebuild where I will first need to determine the balance factor the factory used in 1928 so that I can balance the crank using a modern piston of a different weight. I describe the information needed to do this at this post and this one.