I don't have access to packers, so I'm cooking a lot of flats (they are cheap and plentiful). Before you say packers are easy to get, I'll mention I live in Finland. Even the flats I'm cooking are imports from USA (but seem to be pretty good quality).
I still haven't found a way to keep them moist enough, I've cooked probably around ten in my UDS. They always come up a bit too dry. None of them have been tough.
I've tried hot & fast with foiling, low & slow without foil, all kinds of injections (beef broth, beef broth + margarine) and covering the flat with bacon strips.
I've tried to pull them when they are probe tender, but I'm not sure about that at all. Some of them feel pretty tender (nothing like a pork butt though), some of them don't get probe tender _ever_. I've pulled them at various temps from 188 to 215F.
I've tried to select best looking flats with enough fat cap and also fat on the other side. I've tried to get the most elastic ones. I've cooked them all fat cap down, reasoning that it protects them from the direct heat.
If a flat is dry, is it under- or overcooked? Common sense would say overcooked, but I've found out that common sense and briskets don't have very much in common
I haven't tried using a water pan in my UDS, so that'll probably be something I'll try next. I haven't really liked it with other cuts, though.
Is there anything that could be done better or will the flats always be on the dry side?
I made pastrami once and it was pretty darn good, so that's something I'll be doing again for sure