I believe a push up is 2/3 your body weight. Now that is with proper form. keeping your body staight and your chest just about 1" from the floor on the return. I'm 205lbs so that would be 136lbs. Some even use 1/2 body weight just to be safe.
I would err more to the side of 1/2, maybe even less. I know that with PERFECT form, I can easily do 20 pushups in a row and slowly too, not rushing to push them out.
I'm 190lbs and I highly doubt that if you put 100lbs on a straight bar I could bench it 20 times in a row. Ten definately, 15 probably, but 20? Doubtful.
I was wondering about this not too long ago. I may be wrong, but I figured that if I put both my hands on a scale while in the position of a push-up it would give me a pretty good idea of how much I was bench pressing.
I weigh about 285 and with my hands on the scale I was about 180. That's roughly 2/3 of my body weight.