A string is *not* as accurate as a special tool to measure the dropouts. The reason for this is that the deflection of the string is less along the seat tube and is hard to measure accurately, particularly if the string slips along the head tube.

That said, it is possible to make a much more accurate tool, with a few bolts, some aluminum stock, a hacksaw and a drill. I made one for about $10. Given the amount I use the tool, it doesn't make sense for me to have something more complicated.