1) How much of an erection is needed while jelqing and for example if it is 60% will doing it while any less than that be useless
You have to have enough blood in the corpora cavernosa to create pressure against the tissues as you push the blood toward the end of the penis. Too little blood won’t give enough pressure. Too much can cause injury. The 60%-70% figure is an estimate. Like other things in PE (below) it’s difficult to say exactly how much pressure to use or how much of an erection you need. Most guys find that if they are more than half hard, but not completely hard, then that’s enough to be successful.
2) How much force should be applied while jelqing, as I obviously don't want to bruise or damage my penis
You should be able to feel it get tight toward the end as you stroke the blood forward. That is, you have to clamp down hard enough that most of the blood is forced toward the end. Again, it’s difficult to tell you exactly how much because everyone is different. Plus it depends on how much of an erection you have. The more over 50% erect, the less hard you have to push. Just be able to feel the pressure build toward the end of your unit, then you’ll know some stretching and expansion is occurring.
3) How much force should be applied while stretching the penis
Well, it’s not likely you’ll tear the thing off (unless you’ve really done a lot of weight training). You should pull until you feel the strain deep inside under the pubic bone. I tend to feel it more when I pull right or left and not so much when I pull up or down. Try pulling up, down, right, left and straight out using either an underhand grip (pull with the biceps) or an overhand grip (pull with the triceps). Pull enough that you can feel the burning strain, but not much more than that. Clear? No? It’s difficult. It’s difficult not only to understand, but to explain. You just have to keep at it until you “feel” you’ve got it right.