Is there a formula for marginal difficulty of losing weight / body fat %?

I'm organizing a weight loss competition and there's already a lot of disagreement on how to score it. Those involved vary quite a bit in weight, some only have 10 lbs to lose and others more than 50. Everyone involved appears to be okay with the thought that a heavier person can lose pounds quicker than a person already near their goal weight, but the question is how much easier/harder is it? Is there any logical way to normalize the weight losses (or body fat % decreases) so that the smaller decreases by a skinnier person equals the larger decreases by a heavier person, roughly based on effort? Are there any scientific studies on this topic?