I'm not sure, if that test from satsig.net should be done at the center of the dish, or at the deepest point.
I just read the instructions carefully, to try to understand what they are doing.
With this method, they are quasi sending a beam back in satellite-direction, instead of the other way round.
That way, you'll find the boresight direction for this dish (you mark that on the piece of paper), which has obviously an angle to the dish's face of exactly the offset angle.
Then, with marking the needed elevation angle for a certain satellite to that line, plus an angle of 90 degrees, you get the line angle which you have to make exactly plumb in your dish setup.
A fun method, to do it. How practical it is, I'll leave to your assessment.
Now that I checked what the method is about, I can say: it isn't necessary to place the mirror at the center,
you can place the mirror at any height of the dish,
as all reflections from the dish will go in boresight direction. Don't know why they don't mention this.
I'd prefer simple measuring tape, a calculator (or cosine table), and an inclinometer, for a task like this.
And that's not, just because I don't have a laser pointer.
Greetz,
A33