Child support in Washington is governed by the Washington State Child Support Schedule.
There is not much wriggle room in terms of obligation to pay child support. It is generally a mathematical process wherein both parents' incomes are calculated resulting in a net support obligation to be paid by one parent to the other. Credit can be given to the paying parent when the child spends more time with that parent than is the norm. The dollar amount of the child support obligation is generally worked out based upon the incomes of the parents.
There are other factors that may be relevant
A rough estimation of child support is available at the WA State Dept. of Social and Health Services website: https://fortress.wa.gov/dshs/dcs/SSGen/Home/QuickEstimatorOf course you can also go to an Attorney.