That would load it in favour of the person with the most excess weight I would think.

Ah well, at least it does give all participants an incentive and a support group, so I am sure it will be a positive experience for all of you.

Actually the calculations for measuring how much % of total body weight would not be too complicated and I think it is the method used by "The Biggest Loser" programmes.

Here is someone's BL-style calculator that you could use for the final results, or use it each week entering each week's start/finish weight to see % lost as you go.
http://www.fitwatch.com/qkcalc/calc...percentage.html