Hours, weeks, even months have been spent arguing about Proteus and Portia's route-planning and what its accuracy or inaccuracy reveals. The first thing to say is that above all the geographical explanations, there exists the overarching possibility that Will had only vague ideas of the exact layout and governance of the Veneto and didn't really give a toss whether he was being accurate or not.
Time and distance are both often ruthlessly compressed in Will's work, like an artist's perspective on a broad landscape. To get to Belmont and back for his suit, Bassanio needed Shylock's money for three months. How does this time actually elapse in the play? Not on a journey to Belmont and back, that's for sure. How much more likely is it that Will fiddled with his source material without giving much thought to where Belmont was or how long it would take to get there? The three months were what we would now call 'virtual months' or 'stage months'.
The distance used by Oxfordians, headed by Magri and Roe, to uncover the location of Belmont is twenty miles.
Twenty miles is a virtual distance, which chimes with all the other uses of 'twenty' by Portia and her Belmont chums.