That is correct. If an object displaces 57% of the weight of mercury then that means it's own weight is 57% of that of the weight of mercury in the same volume. This means then that it's density is 57% that of mercury.
You can still use the geometrical method for such a curve. The answer you will get will be an approximation, with the accuracy depending on how well you approximate the curve as made up of little straight segments. But it would still work.