[ReCom]

Won't it be a problem when Current Weight == Goal Weight? (i.e. they achieve their goal weight) Because the way I see it:

Left to Lose = Current Weight - Goal Weight

If Current Weight == Goal Weight then Left to Lose = 0 thus (Weight Lost / Left to Lose) is division by zero ...

I think you should define:

Target Loss = Starting Weight - Goal Weight

Then use:

(Weight Lost / Target Loss) * 100