Well, if it is truly 7/16" EXACTLY (which I doubt, it's just not accurate enough to tell with a tape), that would be .4375". If you're going to be doing precision shooting of any kind, you need to get a decent pair of digital calipers. Most models have probes designed for measuring inside things (like rifle barrels), so this will allow for PRECISE measurment flat-to-flat.
I'm no Whitworth aficionado, but I know enough to tell you that exact sizing is possibly even more important in a hexagonal bore than standard rifling. Buy those calipers (you're gonna need 'em), and as others have said, get hold of DGW, and they will tell you what that resizing die will size to, down to the thousandth.
"Proclaim this among the nations: Prepare for war! Rouse the warriors! Let all the fighting men draw near and attack. Beat your plowshares into swords and your pruning hooks into spears. Let the weakling say, 'I am strong!'" Joel 3:9-10