A crib sheet I found on the Internet a few years back (and apparently no longer up) says:
An X2 would be stronger. I haven't used either in recent memory, so can't add much.#11, X1 (060)
A light yellow-green filter which corrects panchromatic film to tungsten eye response. Differentiates green tonal values in landscapes and reduces distant haze. The filter factor is approximately 2.