Olev, Singles are a different beast to balance and there is a lot of other factors involved in what you want it to do. I called Ernie last fall and asked his opinions on singles and he has done them, But we could get into specifics because until he has the parts in hand it is difficult to speculate.
*One point for clarification for those looking at this. There is blueprinting which is optimizing all the specs, as well as correcting dimensions to the optimum. Some people get REALLY offended when discussing this, but production for any product is done to a std and that can vary widely. Some vintage bikes, especially ones made by companies in financial distress can turn out parts with significant faults or flaws. The Matchless center bearings are a prime example. Brilliant idea but many were misaligned, once corrected or "Blueprinted" they work great.
Balancing takes 2 steps. First is optimizing specs such as modern repop pistons (JCC, EMGO and other Asian pistons for Brit Iron are excellent and 99% of the time spot on in measurements, But I did have one exception recently, But the pins can vary widely in weights. Conversely Old Hepolites varied to the extreme in measurements and weights). So the same with rods, and any other part. You get them to a comparable weight. Then you start doing the math.
At this point you CAN static balance it, and thats better than nothing and some talented people can do this in their home workshop, My old friend Sir Eddy used to do that to all his builds. But a static balance can only get you part way.
For a metaphor or comparison if any of you recall the old static bubble tire balancers many gas stations used to use. Thats basically a static balance. But modern Tire shops now mount up on a machine that spins the wheel & tire and identifies a number of issues. Dynamic does that and more. But you CANNOT check for rocking couple by a static balance. It only shows up when being spun and sensitive instruments pick it up.
extreme rocking couple will rattle your teeth out and blur your vision.
Now, as far as what a BSA might be, Ernie knows the balance figures and can quote percentages but shys away from it because it varies widely. One is, I know for a fact that some BSA cranks for A10s can vary as I have weighed a few,. plus there seems to be a variable in type and part number and I spent a lot of time on this site and Britbike looking at different unit and preunit cranks, (Very confusing) So, while I cannot say conclusively for all BSA A10 & A7 cranks, I can say the Unit A65s were all over the place. We did several including some experiments with lightened and balanced cranks. I have some NOS BSA Flywheels and they made several different flywheels and they balance differently not just in weight but some disparities even in the same part numbers.
**I had an old flat tracker and MX racer known as "Capt Dirt" and he used to build a lot of racebikes and once told me the BSA unit twins were superior in many ways, but their cranks were too heavy. He told me to look up the lightest Triumph 650 crank (1966 IIRC) and make the A65 Crank the same. Not easy, about a 4 lb difference IIRC (I have notes I would have to consult to give exact specs). To get down to that, we had a lot of machining and to balance we had to add mallory to get it to work. Not the optimum street bike but it was VERY fast and revved and spun up like a 2 stroke. It was a super fun hooligan bike but not what you would want for lazy back road toodling about.
So, yes, Long story short, I could get you some approx balance factors but there is too many variables to say exactly why, And before I forget again.. Most repop pistons for BSA twins seem to be quite a bit heavier, and that can skew your numbers. I will be looking at that again when I build my 68 Spitfire.