Checking the preferences of the viewer, I've seen that I can easly create a PW3 display
(see screenshot).
The margin values on top are the ones I use in my ePub.
The page size values are the ones of the PW3 screen (according to MR wiki)
And the viewer has the good idea to keep in memory the last values used here.
To make it short: everything is good.
I can quickly use the viewer to check how the ePub will fit on my PW3 screen.