Since new, mine idles about 800-900 RPM's so it sounds like your adjustment is about right. It also sounds like I'm already doing what most others do, warm up wise, so I'll stick with it, although I'll probably go with a little higher RPMs.
My tractor does have a block heater but I don't want to leave it plugged in all the time and its too far away to trudge down to an hour or so before plow time (which usually is 5:30 AM.) What I need is some kind of remote control that I can turn it on from the house if I know I'm going to need it.
I switched it to Mobil 1 Delvac 5W40 (synthetic) which has made starting easier this winter. One morning it had cleared after an overnight snow and was -10F. Thankfully, it was a Saturday and I did plug it in a couple of hours before starting. Yesterday morning, it was +10F and started first crank (without the block heater and after 10 seconds on the glow plugs) which has been quite normal this winter.
PS: For those of you that haven't had "winter" yet, I think its all in Utah for the upcoming Winter Olympics! I filled my tractor for the 3rd time this winter yesterday. Last year, I only used a tank and a half the entire winter. We've proably got 2 feet standing on the ground. Nearby ski areas have 15+ feet, all since Thanksgiving.