Second monitor connected to M3800 with QHD+ display

I just brought back my new Dell M3800 (QHD+ 3200x1800) back to the office and am attempting to connect it to my Dell U2412m monitor (1920x1200). I'm using a standard HDMI to DVI adapter but after an hour of messing with this I need to get back to work so want to see if anyone else has experience with this. They are all Dell professional products so it seems likely that Dell already has a solution to this. 

Can anyone tell me the recommended settings to get this working for Extend these displays? I've tried the "Let me choose one scaling level for all my displays" option, as well as turning this off. Perhaps I've overlooked the perfect setting but right now either one screen or the other looks either way to big/small or fuzzy.