I'm not sure there is a formula for 3-rail trains. Too many design compromises to make a large engine work on tight curves.
As far as the manufacturer listing the overhang of their engines in the users manual, what curve would you have them use? Just list the overhang for the minimum curve listed for that engine? Or for the minimum curve and all the curve sizes they manufacture larger than the minimum curve? This stuff adds some cost which could be a factor.
There is always the possibility the manufacturer may not want to list this because a lot depends on the track laying ability of the hobbyist. I've visited layouts that had pretty good kinks between curve sections to make the track fit the area, which would negate any overhang measurement.
It might be fun to have a standing topic on this issue; when someone buys a new engine they measure the overhang on the curves they use and post it. After a while, it would be a pretty good database.
