I have never had a problem with using a Lansky on my Sebenza. Where are you placing the clamp? It should be about centered on the blade. Too close or too far from either end will change you final angle.
The Lansky is great for keeping a semi-convex edge, I use the 17 degree setting, follow it up with the 20 degree setting and then lightly hit the edge at the 25 degree setting with the medium, fine and sapphire stones. Strop with a loaded leather strop and the edge will come very close to resembling the factory edge, only sharper :-D I will use the Sharpmaker for touch ups in between sharpenings.
Be very careful if you decide to use a belt to sharpen. Most inexpensive grinders run at high belt speeds, and it only takes a little error to make a big boo-boo, especially at the tip.
WANTED - #38 AMERICAN WHITTLER SFO & OKCA SFO
"Sharp blades are good to have, if Shire-folk go walking, east, south, or far away into dark and danger" - J.R.R. Tolkien