You only have 2 choices of angles with the Sharpmaker. Of course not every knife will be a match for either angle. It is assumed that when the knife does not have the same angle as the Sharpmaker, you just have to keep working the edge on the rods until it does. It just takes a lot of time with the medium rods, it could take several hours.
In general, most knives should be able to take the 40 degree setting without any problem. If you are dealing with a premium steel that can benefit from a more acute edge, then you can go with the 30 degree setting. If you sharpen a knife at 30 degrees and notice the edge starting to chip or blow out, then 30 degrees is too much for that steel--go back to 40 degrees. A premium steel should not chip just by sharpening at 30 degrees.
You can use the 30 degree and 40 degree settings as 2 seperate uses. Or you can use them together a couple different ways. There is back beveling: This is where you sharpen the knife at 40 degrees. After repeated sharpening at 40 degrees, the shoulders of the knife edge become broader causing decreased cutting performance. The remedy is to use the 30 degree setting to take the shoulders down. And you only do this every once in a while.
Then there is microbeveling: This is where you sharpen the knife at 30 degrees until it is sharp. Then you lightly use the 40 degree setting for a few strokes to put a final 40 degree "micro bevel" in the final edge. This gives strength of the 40 degrees to the final edge, but still allows for the aggression of the 30 degrees in the rest of the edge.
Microbeveling has the added benefit of quick maintenance. In your next sharpenings, all you do is touch up the final edge at the 40 degree settings--just a few strokes. After repeated touch ups at 40 degrees, it will cease being a microbevel and the process will need to be repeated at 30 degrees
Hope this helps.
