The 'angled rule' type of digital gauges don't appear to be reliable. I bought one recently, and it's miles off - complete waste of money. I have one "Wixey," angle-box type too (mine is GemRed), and that does work well.
If it helps, I made my own mitre shooting board recently, with the aid of an A3 laser printer. At 600DPI, it can print a _very_ accurate reference angle for you. I simply drew something like this:
and stuck it down with spray-mount onto the surface of my shooting board. The 'focal point' needs to line up exactly with the edge the blade cuts to, and the bottom line parallel with it, but the "V" gives you the exact lines to set up the stops.
I've made the lines too thick in the image above. In practice, if you stick to hairlines (the thinnest you can see), you'll keep the accuracy. It's easy to do, it works really well and it should be OK with almost any printer, although the bigger the better, obviously.
I used Corel Draw, but any package, including SketchUp, would do, as long as it's a vector (drawing) package, NOT a picture editor.
HTH,
E.
