This flew over my head.
I just bought a Lansky too
Ok, let me explain a little better. It's all about triangles (hope you like geometry!). Here is the important triangle highlighted:
The Lanksy's angle measure is where the stone meets the blade. These numbers are necessarily inexact. If you clamp the knife closer to the spine, then the bottom leg of the triangle is shorter, and the distance from where the metal rod goes into the guide to where the stone meets the edge (the leg of the triangle going from the top left to bottom right) is shorter than if you clamped it further towards the edge. Here is a picture showing the triangles:
The blue legs are an example triangle of clamping it closer to the edge. More of the blade is hidden in the clamp, so the bottom leg is shorter, and the top leg has to be shorter. The orange legs are if you clamp it more towards the edge. In that case, more of the blade is out past the clamp, making the bottom leg longer and the top leg also. Basic trigonometry shows the orange triangle needs to have a more acute angle (thinner edge), while the blue will result in a more obtuse angle (thicker edge).
So, where you clamp it matters. The reason it matters is the length of the legs of the triangles change depending on where it's clamped. That is the basic idea of what I was talking about, but for a different reason than how close the clamp is to the edge of the blade.
Here is the clamp from a different angle. Think about how the previous triangle looks from this direction. The top and bottom legs will overlap, while the vertical leg would appear to be a point.
Pretend we can't move the clamp for a second, so we can compare what the Lansky does to an edge when the clamp isn't moved. The shortest top leg (go back to the previous triangle pic--I'm talking about the diagonal one from top left to bottom right) is right through the middle of the clamp to the edge. Scroll down to the next pic for a second to see it; it's the red one (it looks crooked just because I took the pic from a stupid angle, but notice how it goes right through the middle of the clamp). Now, any triangle with the legs either left or right of that red one are longer (this is a result of the Pythagorean Theorem, if you want to know the math behind it), which, just like before, means the angle is going to be thinner. Scroll back down to the next pic and look at the blue line. Even though it's on the straight edge, it will be a slightly thinner angle there. Honestly, it's only very slightly thinner, and there is so little difference in the edge angle that it really doesn't matter. Where it really matters, as this thread shows, is along the curve of the blade. By the time we get to the curve, we're so far off center that the angle does change quite a bit. The green line in the pic down below shows that.
Now just for fun, I put a dot where the Lansky guide hole might be and measured the straight line to the edge of the blade, one to the point of the blade nearest to the handle, and to the tip of the blade. The results are less than surprising, with the straight line being shortest (making it the thickest angle) and the length to the tip longest (making it the thinnest angle):
So, since the angle stays most accurate at the straight line through the middle of the clamp, what I suggested before (and what I do with big knives) is to reposition the clamp to the center of the curve of the blade to reprofile the edge there. Doing that will make it as close as Lansky-possible to the same size angle as the straight part of the blade. Remember that the distance from the clamp to the edge also matters, so you want to clamp it the same distance as you had it before you moved it.
For really long blades, it might be worth moving it a few more times, but if you aren't OCD about your blade angle and/or aren't having chipping/bending issues, just clamp it right in the middle of the blade.
/ :foot:
have fun..
