Surely there is a better way?
There is no way that I know of to do this automagically, but there is some improvement possible.
Instead of choosing User Defined in the Print Setup screen, choose Properties, and enter the printer properties screen.
Choose User Defined here, next to the paper-size control.
In the User Defined Paper Size screen, choose a name for your new size, set the size parameters, and save it. Then exit the printer properties screen and the Print Setup screen, then enter the Print Setup screen again, and choose your new paper size.
At this point, I normally set up the rest of the print properties for this size and paper and save a preset in LR Print module.
Obviously, this procedure requires an iteration through the printer properties screen, back to LR print module and then back to Print Setup.
I hope this helps,