Good question and good answers. It come down to experience and what you feel even for some time after holding a stretch. You need to learn to differentiate between the feeling of skin stretch, lig stretch and tunica stretch, before you think about power. Try simply stretching the skin with simultaneous grips near the base and the glans held lightly and pulled apart if you don’t know what this feels like. Try a BTC stretch to put most tension on the ligs (thats what retsnom is talking about with the feeling near the base with the fowfer). Try a straight up (OTS style) stretch to concentrate strain on the tunica.
Once you’ve identified these, a good lig stretch may create an intense warm feeling near the base and in the body above the base. It might be worth trying a few JAI’s to isolate the feeling (if for no other reason). A good tunica stretch will give a feeling of almost throbbing warmth through the shaft. These feelings should be perceptible some time after a stretching session.
Initially you should not need to provide much force (be careful not to pull too much) but very soon you’ll be wanting to look at way of increasing the strain (v stretches, JAI’s, blasters).
You don’t need to pull as hard as you can, simply create a mild tension, then try increasing it slowly until further increase seems to have no effect on the feeling of the stretch. Its always best to tension slowly into a stretch and relive the tension slowly at the end.
A fair number of people do piss pulls (stretches during the day when using the toilet etc). Its hard to say how effective these are but its probably fair to say that the more time the penis stay in an extended state the better. To this end some people use ADS (All Day Stretch) devices. Check the tutorial forum for lil2big1’s demo.
You are quite right about the nerve bundle location. Just about any form of PE can have a negative effect on erections but how much of this is perception and how much is reality is a good question. To check for certain that you are getting night time erections do the stamp test (take a row of postage stamps and fix them around the penis in a flaccid state, if they break in the night its a good indication of night time erections).