There are lots of answers here and mostly from real estate experts, but maybe I can help.
Some assessed values, regardless of where the county got them from, are just plain wrong. Some are low and some are high. If you believe yours to be high, be prepared to have proof.
I could not find a section in King County's website where you can get the petition form. I got a digital copy of the form and actually held a class on how to fill it out properly. It's not easy, but it can be done.
I didn't know how to post a document on the web to be downloaded other than my member profile on Mortgage News Daily. I've referenced a link to my profile where I host several documents including the King County Property Tax Petition form.