Well, every judge will do it differently, but he could rule based on the current living situation or he could rule based on what is currently filed. If the last thing filed says you have 50/50 residency, then he "might" rule based on that. The 20% parenting time adjustment went away in 2012 with the new equal parenting time formula instead. You can search around this forum and find many threads on exactly how that new formula works, but it will probably be a drastic reduction from the $600.
However, if you actually have residential custody, mom should pay child support minus a parenting time adjustment. If mom has 20% parenting time, then I believe the adjustment is 5%. So instead of paying $600/mo, you could be receiving $200/mo. The way the formula works, is it's mostly dependent on the paying parent's income, not so much so, the receiving parent. But, both incomes are entered into the equation. We offer a free child support calculator on this site which does a very good job and is easy to use. You can download it here:
http://kschildsupportforum.com/kansas-child-support-calculators-and-forms/kansas-2012-child-support-calculator-free!/msg879/?topicseen#msg879If you don't have an order through the court (even an filed agreement) that says you have primary residency, if you file a motion, there's a chance mom could fight you on that. Typically the way things work in court is one parent files for child support, the other files for some change in parenting time/custody (or vice versa). This occurs all the time. So, when you file for child support, the judge may not honor the current living arrangement. So, to ensure that you would win your case, you should probably talk to mom about signing an agreement for the child to live with you. No mention of child support would be in that order and mom believes she would be losing the expense of the child while still getting $600/mo. Then 30 days down the road, you file a motion to modify child support. An attorney may want to take all of this up at the same time, but I'm a firm believer in putting all the steps in place first, then your final motion will be a walk in the park.