Well, one thing you should note is that 64% is the standard percentage of the upper-body compared to the entire body.
On the other hand, a sit-up is pretty hard to measure. A body-builder who has huge upper-body muscles compared to a long-distance runner who is fairly skinny and has an even distribution of muscles (maybe more in the lower-body portion) are going to lift different percentages of their body weight.
So, sit-ups are kind of hard to determine exactly how much weight you are lifting. But, you could lay down on a bathroom scale, with only your upper-body resting on the scale. This way, you could approximately measure how heavy your upper body is, and how much weight you are lifting.
Hope this helps! :D