Where itime is the integration time per exposure in seconds, expnum is the number of exposures (must >=4, 1x ABBA), Kmag is the K magnitude of the target, and seeing is in arcseconds.

For example, four 600s exposures on a K=10 target with 0.6" seeing will give you:

The same target with the same exposure time but in 1.0" seeing will give:

Typical seeing at McDonald observatory is ~1.0".