Its been a while since gray cards and stuff are introduced to help photographers hone in on getting the right White Balance temperature. Frankly, it was more of an art prior to this invention. However, in the recent development of "lens cap" version of such WB tool, a company with a product called ExpoDisc seems to nail this art work more towards science and predictability.
In a way, experience in measuring K temperature is being superseded with high-tech gadgetry. So, for a mere $69.95, your skills in shooting better-balanced images is acquired.