Hehe I was thinking earlier today "I wonder how he is managing to get the perspective so well. Probably just winging it." So I bet it's fairly tricky

Hmm have you tried disconnecting the second monitor to see if the tablet functions correctly without it? Here's how my tablet mapping is typically set up (but it also works even with both screens mapped. The tablet just ignores the black area you see in the little example window there):
tablet.png
If you play around with the "Force proportions" and "tablet area" options (or their equivalents in the software you are running) it may help. Also I generally find "windows ink" can sometimes create some bugginess you might want to try turning it off if it's on.