Here’s a paper that explains how to derive that magic constant and others depending on what measure of error to minimize in the sqrt approximation. http://www.

Here’s a paper that explains how to derive that magic constant and others depending on what measure of error to minimize in the sqrt approximation. http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf