No, it's not the square root except in the case of filter factor 4, which is a 2 stop correction.

It's actually 2^s = ff, where s=stop correction and ff= filter factor. In other words, two to the power of the stop correction = the filter factor.

2 to the power of 2 = 4

And 2 to the power of 3 = 8 (2 x 2 x 2). (8 is the filter factor, 3 is the stop corrrection)

It's best to find a table with the equivilants, or make one yourself with a calculator that will do roots. Finding 2 to the power of ? = 5 (for instance) in your head ain't easy. Or 2f/2f's method will get you close.