I am not aware of any formula in common use by Sorby spiral tool users and I have seen several demonstrators including one from Sorby. Most folks suggest not setting the angle setting more than 1 1/2. I would not want to spiral on too much surface on a curve bowl. In other words if the...
I have one and love it! It is one of the more popular upper end lathes in the USA. You should be able to google the term mustard monster to find the link to a web site for Powermatic 3520 user tips and tricks.
I used the saw horse technique but three old guys can safely roll the lathe bed up...