How can I deduce at what angle my knife should be sharpened against glass stone?
The &quot;sharpening angle&quot; of a knife is the angle between the sharpening surface and the blade. The 'original edge' is the angle at which the knife has been sharpened previously. Sharpening at close to/slightly below this angle will make sharpening the blade easiest, as you don't have to work hard to change the angle (which requires moving more material from the knife).
I could not find any info on the factury angle for the knife you mention, but most Western-style chef's knives are sharpened at an angle of about 20 degrees. Note that this is a &quot;double-bevel&quot; knife, so the angle will be about 20 degrees on both sides of the blade. See for example this page for some graphics and more info.
The original angle is not something you calculate. Either you find reliable information from the manufacturer, or you have to somehow approach this angle by trial and error. A 'trick' that I've seen some knife sharpeners do is to 'drag' the blade over the sharpening stone, slowly decreasing the angle until there is a change in how much friction you feel.