I think jelking 80-90% erect is more effective. The 10-20% space is the area you are stretching when pushing blood by jelking. This continuous stress eventually stretches the tissues and enlargement takes place. If fully erect, the blood doesnt have anywhere to go, so jelking effect is minimal compared to 90% erect. You should allow some space for expansion. I lost some girth by jelking 100% erect. I gained it back when I reverted to the 80-90% erect jelk.