Using a linear function (like multiplying by 0.03333) works well in the 3-6 rep range, but not as well outside that range. Almost all the calculators I've seen use a linear function.
The best numbers I've seen are Beachle and dos Remedios' numbers at Predicting One-rep Max. Ignore the...
Just so people don't get confused, your math doesn't quite work out. If you're 16% body fat at 206 lbs, then at 203 you're about 15% body fat, not 10 %.