A 60 degree tool is the "broadest" I would use and even better is one made much finer. I can't think of a vintage gun with the checkering made with a 90 degree tool.
Isn't a 60 degree tool narrower than a 90 degree tool? It will leave 60 degree peaks which are steeper than 90 degrees peaks.
I don't know of any ready made tools that will cut broader than 90 degrees. I imagine you could make 120's or so but they'd be very hard to keep in a groove.
I agree, stop when you begin to get tired and make mistakes. Go back to it after a nap.
