Even on a stock bike the oil level might read off a bit if you are not on a level surface and up on the center stand. The book calls for it to be up on the center stand to check the oil.
Now on a hardtail, at an oil change I would add exactly 2 liters, or 2.1 quarts. With the bike standing as straight up as I can get it. Warm it up, shut it down and check the level on the dip stick. Wherever the level reads on the dip stick is where you want to keep the level. If you can see it in the window, fine if not, fine.
On stock bike on the stand the oil level if on the stick is about 1/2 way between the marks, then the window will show it at about the top mark.
On a hardtailed bike the rear is lower. This makes the engine tilt back, this makes the oil settle toward the rear of the engine. The window is in the front. If the tilt is enough you won't see oil in the window and the proper amount of oil will read higher on the stick.
Try putting two 2x4's or 2x6's under the rear wheel, then check the oil on the dipstick and in the window.
Leo