The court has discretion to deviate from the standard child support calculation for significant residential time, as noted in section 3.7 of the Order of Child Support, and authorized by RCW 26.19.075(1)(d):
(d) Residential schedule. The court may deviate from the standard calculation if the child spends a significant amount of time with the parent who is obligated to make a support transfer payment. The court may not deviate on that basis if the deviation will result in insufficient funds in the household receiving the support to meet the basic needs of the child or if the child is receiving temporary assistance for needy families. When determining the amount of the deviation, the court shall consider evidence concerning the increased expenses to a parent making support transfer payments resulting from the significant amount of time spent with that parent and shall consider the decreased expenses, if any, to the party receiving the support resulting from the significant amount of time the child spends with the parent making the support transfer payment.
There used to be a residential credit formula which was repealed in 1991: one year after it was enacted. That old formula is still included in the most widely-used software program used by courts and attorneys to calculate child support. Its ease of use (pressing the button) leads many attorneys to still use it to aid the court in the exercise of its discretion.
If you don't have such a button, I found it hard to explain how the calculation was performed. So I looked up the old support schedule and worksheets. The 2007 child support schedule workgroup collected a prodigious amount of child support information online here: http://www.dshs.wa.gov/dcs/Resources/WorkgroupMaterials2007.asp
Among other things, it includes the previous versions of the child support schedules and instructions and worksheets (under the category Washington State Child Support Schedule Historical References, click the link for 1990 Support Schedule.) The 1990 support schedule's Worksheet B has clear instructions on how it was calculated, using the old 25% (91 overnights) threshold. However, a legislative colloquy in 1991 rejected that threshold in support of 35% (128 overnights) – but remember: there is no statutory formula under the law today.
In fact, some recent court decisions have expressly ruled that basing a deviation on the old formula is error, and that instead the increased/decreased expenses must be considered (as provided in the statute now) in order to grant a deviation for significant residential time.