RCW 26.19.075 provides standards for deviation from the standard calculation. The statutes are here: http://apps.leg.wa.gov/rcw/default.aspx?Cite=26 .
Whether the judge in your case would agree to the deviation depends on the facts.
The free online WA child support calculator is at http://www.courts.wa.gov/ssgen/ . You can use the calculator to estimate what the standard basic child support would be in your case.
You may want to review your facts and options with an attorney.