80% for one person may feel significantly harder or softer for another, so even a percentage scale is not really a good relative measure. Since the best erection level will be different for everyone, you should try to find a level which literally feels the best, that is, the most pleasurable, and I’m not talking about the kind of pleasure from masturbation - when jelqing is performed properly it produces a pleasurable sensation in its own right. That is the feeling of blood being squeezed through the CCs.
Your penis should be soft enough for your OK grip to compress the tissue significantly, so that when you start the milking motion, you can feel that forced perfusion of blood moving up your shaft as your grip rises. The greater that initial compression, which will be limited if you’re too hard, the deeper the tissue which you’re flushing with fresh blood will be, and the more effective your jelq stroke will be. However, if you’re not erect enough, there won’t be enough blood around to really be forcing it to go anywhere, which is no good.
So you must find a balance between being able to compress the tissue as much as possible with your OK grip, and making sure there’s enough blood around, which may be reflected by how good the stroke feels. Frankly, it is not easy getting it right, and many will struggle with their erection constantly becoming too hard or finding it difficult to remain soft enough. Don’t worry. As you become more experienced, your penis will start to learn that jelqing is a process in its own right and will begin to stabilize at the desired level.