I have a railboard which seems to have a natural thumb position on the "keel"

But I seem to find myself putting my thumb on the edge (I suspect this comes at least partially from a classic[al] guitar background and I'm used to addressing the strings with a bit of finger arch. In both my guitar and gamba experience I've learned to not use my thumb as a "counter pressure" brace to I'm not leveraging into my thumb)

now I realize the stick is an entirely different beast, so my prior training may be working in my disfavor...

any thoughts on this issue? where should my thumb "hang out" by default?