Appropriate Age vs. Weight Table?

Well I was browsing through different BP forums and alot of advice concerning BP's requires weighing it on a scale. This includes everything from parasites, going off feed, and recently, "waste disposal".

My problem is that I've never seen an Age vs. Weight or Length vs. Weight table. So if for example someone has a 24" snake that ways 1500 grams, is this good? Is this bad? What if it's 2500 grams?

What do we determine what are safe and appropriate weights to keep our pets at?

I am inquiring because I've seen owners advise only to worry about a snake that has gone off-feed or sick if it has lost weight. Well what counts as "lost weight", 10 g, 100g, or 500g?

If there isn't such a reference, does this mean that I should start collecting data on BMI, weight, length, and age and come back with an excel spreadsheet in a few months?