Right now TC sums up file sizes and assumes that copying speed is linear with it. It's grossly incorrect on small files and even more on a mix of big and small ones. Earlier today I've had a case when after doing 1/3 of work, TC showed that 2% was done.
I think it would be better to do it in the time scale, because adding X bytes would be very arbitrary. Assume some seek time and bandwidth and preferably adjust them as you copy things.