At the college and increasingly at the high school level, coaches are telling athletes they are not allowed to do cleans. I was fortunate enough to go to a high school where it was allowed, but my college coaches said I couldn't. Their reasoning is solid - the lift is very dangerous and hard to master. I had perfect form and set records at my high school but got cocky one day and couldn't do squats or cleans for 2 years after that. I would say the perfect weight is 100% determined by your form. If your form sucks, back off the weight and work on that. Your body will tell you what you can do when your form is spot on. As far as a bare minimum, PillarofBalance is correct. Another easy indicator your weight is too light would that you are hitting yourself in the mouth with the bar as it travels upward and before your "catch" it. Incidentally, with a hang-clean at least, that is also an indicator that your form is good.