How can I deduce at what angle my knife should be sharpened against glass stone?

The "sharpening angle" 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 "double-bevel" 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.

I'm afraid you are asking the wrong questions here. But let's try answering.

1. "The original edge" in this case is a slightly shortened formulation for "the angle the knife's edge had when it was originally produced".

2. You don't calculate the original edge. In principle, you could try finding a data sheet for the knife or calling the producer to learn the exact number, but that number is quite useless when sharpening.

3. I can't provide you with an exact number, and while I have read the writings of knife afficionados who care about that number, it seems that the exact number is a matter of personal preference for them. Anyway, knowledge of this number is also quite useless when sharpening.

To answer the question in the title: you don't deduce the angle. Deducing is a cognitive process. Sharpening is a motor skill. A person who can sharpen knives will lay the knife to the stone and recognize the correct angle by look and feel.

A person who is just now learning to sharpen knives will, of course, not be able to recognize whether the angle is correct. They - you - will just have to hold it at a roughly okayish angle and try sharpening. At this stage, you won't be able to keep the same angle for the duration of one stroke anyway. And the commenter you are citing really found the right way to describe how to recognize the "roughly okayish" angle - you have to aim to be at a slightly steeper angle than the angle of your original edge.

Take that comment literally and apply you while practicing. To start, take your knife, hold it at the stone in a position to sharpen, at an arbitrary angle, and now slowly change the angle in both directions while using your eyes and the physical feedback of the knife in your hand to decide when it is closest to the original angle, then make it the tiniest bit steeper than you can. Then try sharpening at that angle. You can repeat this adjustment process as frequently as you need, even for every stroke, until you notice that you can match the angle without needing it.

You can shorten your total learning time by finding a teacher who is sitting physically close to you, observes you, and can recognize when you are holding the knife at an improper angle. But a protractor cannot do that job.

Nowhere in the practice process is knowledge of a number helpful, unless you have some rainman-rare savant ability which lets you exactly visualize angles to sub-degree resolution when told their number, and hold something with your hands at the angles specified by that number. If you have that, you already know the original angle