What exactly is "blade tilt" in relation to portable bandsaw mills?
I believe it is the angle of the blade to the end of the log, or the side of the bed. Usually it is 10 to 12 degrees. This prevents the blade from entering the end of the log squarely. This will help improve blade and engine life.
This tilt gauge came with the mill, when I got it from Wood-Mizer. You attach it on to the blade between teeth so the set of the teeth doesn't push on it. Then you measure from the front end of the gauge to the bed rail and then move the saw head forward and measure from the bed rail to the back end of the gauge.
In the photo below, the framing square is being used to measure from the bed rail to the gauge on the first bed rail. You can measure to the bottom of the gauge or the top - it doesn't matter as long as both are measured the same. The distances should be the same for a level cut. If the distances aren't the same, you need to adjust your guide blade roller bearing so that they are the same.