Elementary mathematical ecology Book

This is really about ballpark estimations in general, if it's the one I read. The title comes from a story about a farmer who wanted to improve his farm--so he wrote to the local university asking for advice. He got various responses, but the one from the Math department began "Consider a spherical cow...". Cows, of course, aren't spherical (more cylindrical), but the need to use such thumbnail esitmations is found in more fields than environmental studies, and is an art worth learning.