Great question, Kromm. I think this is particularly important after last season, when we finally had a winner go too far with the weight loss.
If there's going to be a grand finale - and I suspect the producers and NBC want that - there has to be something measurable at that particular time. If not percentage of weight loss, then what? Is there a gauge of general fitness that can be used in addition to or instead of percentage of weight loss?
The simplest change would be something like "greatest percentage of weight loss without going below a given BMI." BMI isn't hard to calculate; the contestants could do it at home as they weigh themselves each day. If there's a tie, TPTB can determine in advance if the prize is to be shared, if there's a tie-breaker, or what.
Another option would be to have that plus another gender-neutral measurement or two - percentage of fat, ability to perform athletically against a baseline test given on the first day at the ranch, nutrition quiz, something.
Logistically, I think there must be a grand finale in order to have suspense build through a given number of shows, with ratings highest during a grand finale that occurs during sweeps. TPTB and NBC are in the business of pulling in advertising revenue, which is done through ratings. So a "who reaches X first" gauge will not work if it can be reached in too few episodes.
I get tired of Jillian and Bob, but it took the show a long time to find a "keeper" new trainer in Dolvett, so I'd rather keep Jillian and Bob than bring in another dud like Kim.