I don't know of any way to calculate shaft size using only GPM and head. There are some very good reasons to calculate shaft size though. The biggest one is to minimize shaft deflection. Depending on the type of pump, this could be the single most contributing factor in seal and bearing failure...
One of the things you asked for was an opinion from a "pump expert". In my experience, if you get 5 "experts" to respond, you'll get 5 different opinions. There is no one right answer. When I work on a system, I try to keep it simple. You state that the Roth pump was an attractive option, but it...
