How hard should you be while clamping

Some say 100% some say 80-90% erections level

Remek has one of the more authoritative guides here. I would follow its recommendations.

By the way, I noticed your join date is December 2006. Perhaps you started PE long before you joined here, but if not, you should really spend some more months conditioning your penis with lighter exercises before trying anything as potentially traumatic as clamping.