Some of it is pure opinion, and some of it is experience. I don't believe some numbness is to be accepted. It occurs for enough people to be formally "the norm," meaning most people have some, but I think it can be beaten.

First thing is to start with your saddle at the standard height. I won't say it's "correct" or "normal," because most of us make small adjustments away from this position. One standard starting point is to measure your cycling inseam of pubic bone height when barefoot (there used to be good instructions on this at http://www.rivbike.com/kb_results.asp?ID=39. Measure in millimeters if you have a metric tape, or to the nearest sixteenth of an inch. The precision is important. Once you have this number multiply it by 0.883. With your saddle level, set you saddle height (measured from the center of the bottom bracket to the top of the saddle in line with the seat tube) to this value. You'll probably find some good reasons to deviate from this setting, but it's normal to see if it works. For most people this setting also gets your knee bend and range of motion into the correct range.

Then get on the bike sitting as comfortably as you can and have a helper hold you steady, or rig up a prop of some sort. With your pedal forward, drop a plumb line from the front of your knee. It should pass just in front of the end of the crank, or up to 2 cm (about 3/4") behind it. Non-competitive riders (we know who we aren't) usually do better with the knee toward the back of this range. If you set it at the forward edge of this zone, it's called Knee Over Pedal Spindle, or KOPS. Usually those four letters start a discussion. Move your butt forward or back ward until your knee is in your chosen position, then get off, loosen the saddle-seatpost clamp, and slide the saddle fore or aft until it is able to hold you comfortably in this position. Some trial and error will be necessary. After you're satisfied with this adjustment and the saddle is again level, re-check the saddle height measuring to the same point as before. You may need to raise or lower the saddle a bit to accommodate the sliding forward or backward. If everything is secure, go for an easy ride and notice how it feels.

This is more or less the standard approach to saddle positioning. There's a lot of fine points to troubleshoot aches, pains, abrasions, numbnesses, et cetera. None of this is claimed to be what you or anyone needs, but it's the usual starting point. See how you do with this, as a starting point.

I think learning how to fit is a matter of getting the little bit of knowledge and then to see how tune yourself in, making small adjustments at a time.