Here is a great trick from another forum on figuring out the edge angle when sharpening.
It can be found here,
http://www.chadwrites.com/knife-sharpening-coin-trick-magic-angle-finder/ , but the main part is shown below.
Knife Sharpening: Coin Trick & Magic Angle Finder
Here’s a quick Technique of the Week to get us started. You often read that you should sharpen your knife to 20° or 15° or some other seemingly arbitrary number. If you have a standard European-style (German, French or something along those lines) chef’s knife, it probably has edge angles of 20-25° per side. A western-style Japanese chef’s knife (Shun, Mac, Global, et at) will have edge angles about 15-16° per side. The trick is being able to find and repeat those angles. That’s where the Magic Angle Finder comes in. I won’t go into the math, but you can divide the height of the blade (at the heel) by a specific number to find out how high to raise the spine above the sharpening stone for a given angle. It sounds more complicated than it really is. Here’s how it works:
Chad’s Magic Angle Finder
For 20° divide by 3
For 15° divide by 4
For 12° divide by 5
For 10° divide by 6
For 8° divide by 7
So, if your chef’s knife is 1.5 inches tall, measured from spine to edge, and you want to put a 20° edge on it, just divide 1.5 by 3 to get .5 inches. Lay the knife flat on the stone and then raise the spine .5 inches. You have just set the edge angle at 20°. It works in metric, too. If you want to put a racy 12° edge angle on your Japanese gyuto and it is 45mm wide at the heel, just divide 45 by 5 to get 9. Raise the spine to 9mm above the stone and you are good to go. It can be a little tedious to sit there with a ruler, checking to make sure you’re at the right height but the payoff in consistency is well worth it. It also helps ingrain the angles into muscle memory so that the next time is much easier.
Coin Tricks
If that is too nerdy for you, you can just use a stack of quarters. A US quarter coin is officially .069 inches thick, though they vary slightly. Four quarters is a little more than .25″, five quarters is about .33″, six quarters is about .4″, 7 quarters is just under .5″ and eight quarters is just over .5″. It takes a surprising 15 quarters to reach a full inch high.
To raise the spine of your chef’s knife .5″, use 7 quarters. It’s not quite .5″ but the width of the spine makes up the difference and puts the spine at almost exactly the right height.
