The problem with this is that it's very hard to see if it's front or back focusing. I don't have enough space to test it properly at 600mm (need about 30m or so), and when I do try and test it, pretty much the whole image appears as out of focus, like the original measuring stick one I posted above. Cheers
I've never tweaked the 150-600 but tweaked a few other Sigma lenses. What I did was find a sufficiently long row of fencing. I put some white tape on one of the fence posts and focused on that. Then I could judge front or back and knowing the spacing of the posts helped me to quantify the issue.