Extinction-o-Meter

Extinction-o-Meter is an utility that can apply corrections to visual brightness estimates of variable stars, so that differential atmospheric extinction is taken into account.
You can either supply the extinction coefficient (K), or you can determine it using visual brightness estimates done on stars of known magnitude.
Specifying comparison stars for the brightness estimates is done by selecting them from a chart; you'll need to supply the variable star name, the limiting magnitude and the size of the field of view.