TomD
I think you have the right idea. However, if you read my notes on layout you will realize that you draw a line on the side of the dai, that is the plane's angle, say 45 deg, and that is the top line, the bottom line is scribed from the plane blade held on the side of the plane. BUT the plane is held back about a third of the top line, so the edge is say at 1/3rd horizontal line to the base. The blade is only two thirds of the way in. So while minor errors in the blade fit are critical, this initial process is not at all critical, because you have to ink the blade into final position.

What is critical, is that the top line is clean. You do not want to have to adjust it, though you could, but this is virtually the only line where it really pays to have it in "right". Now 44 or 46, who cares, but both lines parallel to each other, and square, that would be good. Not the end of the world if it doesn't come out right, and fairly easy to get right, but worth a little effort.

If you cut freehand, you want the flush cut edge up, if you cut off a wedge you want it down, assuming you are using the LV ryoba.