I came across this thread via google, it's got some good advice in there.
In the funny way that it happens, I became convinced that this angle was my issue. I've tried sharpening (after measuring all my teeth and getting them all pretty close to one another, and spending a fair amount of time getting a good hook and chisel on them), setting depths, etc etc using all the advice I could find, and I just couldn't get it to cut well in this wood.
After reading this thread, I went and ordered myself a DAF because I need one in my shed for other things, but in the interim I did up a quick little reference on paper. Photos attached. As has been pointed out, 6 degrees is approx 1 in 10 (i.e. Tan(60) = 0.10510, which is about 1 in 9.5, conversely atan(0.1) = 5.7 degrees). So I ruled a line that was 200mm long, and a line perpendicular to this that was 20mm high, and then drew the angle.
I had my chain off the bar, and held it taught so that the tops of the teeth were on the line, and looked at where my raker depth was at compared to the line. Lo and behold, actually pretty close to 6 degrees.
If you want to be really precise, just vary the dimensions to the correct ratio for whatever angle you want to measure.
So the angle isn't my issue. I'm coming around to the fact that I'm just trying to cut super dense, hard, old wood that's a bit dirty, and it's just going to be hard work. These are 100 year old stumps from my house, I believe they are grey ironbark. Chopping into 250 to 300mm lengths to split for firewood.