Weight Loss - A question of maths???

I'm entering a a bit of a comp with a friend over who can lose the most body fat in 2 months.

Not weight - body fat.

We are being quite precise and are using skin-fold measurements from a dietician friend on 9 body sites.

Enough of the trivia.

Here's the question.

I am 52 with 20% b/f with a weight of 99.8kg. He's 40 with a b/f of 14% and a weight of I think 79kg.

We both recognise that a straight equalled percentage is probably not fair 'cause the lower the b/f you are - particularly at these levels - the harder it is to lose. And...I have more to lose than he so it may be easier for me.

On the other hand I am 12 years his senior and that may mean I have a harder job to lose the fat.

We were wondering what ratio one would use to normalise the weight loss requirement of each of us to control for the discrepancy of the b/f at the start and to make this comp fair??

He was mentioning divide something by something and gets I think 1.6 - (dont ask me???!!) saying for every percent of b/f he loses I have to lose 1.6%

Forget the age - I'll have to wear that.

Any suggestions from math guys/gals????