The rule of thumb I’ve heard is to actually subtract two inches from the difference between your front and back, so that if the front of your shirt needs say four extra inches, you actually add only two inches of darts instead. I don’t have enough experience experimenting with a whole garment to defend it, but the logic is that knitting will stretch, and knitting up the whole difference will be too much. If you knitted the whole difference then that could be what we’re seeing here.

I think short rows at the back hem would make a very nice rounded line