The method of choice seems to be to test all these methods (Tanita scales, calipers, military charts, expensive hydrostatic testing) and use the numbers that are most flattering. :) Depending on which method I use, I may be at 8% or at 14% body fat. Hm....

The truth is, it doesn't really matter what you use. The best and cheapest method is to look in the mirror (and take monthly progress photos!). If you like what you see, you're on the right path.

All methods come with built-in margins of error and are based on (sometimes false) assumptions and generalizations about human bodies. Read Mark Sisson's post about how his BF% came back at a ridiculous 17%. If you look at his photo, you'll see that he's probably closer to around 8%.

Sisson was using the hydrostatic method, the so-called gold standard of bodyfat testing. Why was the number so inaccurate?

when I researched how they actually got the original data they use to estimate body fat, I found that it was largely from autopsies performed in the 1860’s and 1870’s. Not many autopsies have been done for that purpose since. Also, the reference data on skinfold tests and hydrostatic weighing still assumes that as you get older, you automatically lose muscle (regardless of how you eat or how much you work out)

Whatever method you use, you'll get a number. It's just a number! Depending on your body type, age, diet, exercise habits, temperature, skill with calipers, hydration status, the number could be wildly off. Don't worry about the number too much.

Now, any method is useful for keeping track of TRENDS in your body fat. If you see that number increasing every few months, it's probably not a good sign. If, on the other hand, your number consistently creeps down, then you're probably on the right track.

So, my recommendation is, go cheap. Use the mirror. Or a measuring tape and the Navy charts. Or a $6 pair of calipers. But don't stress about the number too much. Just use the number to track your progress over time.

As an aside: if you want to lose fat and stop being skinny fat, my recommendation would be to run a LOT less but start incorporating more resistance training and a bit of sprinting. 21 miles per week is probably way too much for you and will probably discourage fat loss at the expense of muscle loss. (Great job on getting to 3 miles per day from not being able to run around the block, though! You should be proud of yourself.) You can search PaleoHacks or other sites for advice about this (Google "high-intensity interval training" or "CrossFit" or search Mark Sisson's site for "Primal workouts"). The PaNu site talks about this a bit as well, I think. But start slow and build up over a few weeks, so as not to injure yourself in the process.