I notice that the Minwax Wipe-on Poly sells for more than the standard
brush-on Poly (in part becomes it comes only in smaller cans).

My understanding is that Wipe-on Poly is just regular polyurethane
diluted with mineral spirits.

Can I do just as well by making my own Wipe-on Poly from bulk
polyurethane by just adding mineral spirits? (Or are there other
"magical" ingredients that are added...)

If so, what ratio should I add it in?