Those "rules of thumb" are general....
I'll let you know how I measusre mine.
I get the longest genoa and hoist it. I sheet it to one side, then I measure the distance form the clew, forward of where it has to pass, either a mast or a stay (if we are measuring the front most sail), then form that point to the genoa track car, then to the
winch...
I then add 3 meters or 9 feet. There's you sign!!!!