If I want to have a good working edge on a knife how sharp is sharp and how do you test an edge to know when you are there

tough question;  depends on the skill of the user, personal preference, use of the knife.  "scalpel sharp" at one end of the scale, "axe sharp" at the other - - - I have my preferences which differ from what my friends like.   I personally keep most of my knifes shaving-sharp, but also keep some wedge-edged for rough stuff.  Too fine of an edge will break or roll with some uses, be just fine for others. I keep different edges on the two blades of my pocket knife;  one razor sharp and the other more hefty and use em as the need arises.

I keep all my knifes razor sharp (which, for me means this cut hair off your arm). It's just the way I like to have them.

Shaving sharp...generally.

You can get a 90 degree angle shaving sharp but dont have all of the cutting qualities you need. Besides heat treat, edge geometry is vital to the cutting of a given type of tool. Just thinking outloud.........