I think the best you can do is to do one of two things.
1. If her income for working 30 hrs a week is less than minimum wage, I would ask that minimum wage be caculated. If she recently reduced her hours, I would ask that it be calculated at 40 hours at minimum wage.
2. If she had a different job and was making more in the past couple years, you (meaning your attorney) can ask for her tax returns for the time she worked at the other job and say she is not working to her potential. That way you can get her income imputed.
Honestly though, to fight to reduce your CS will likely cost you thousands of dollars and might cost you a lot more emotionally. You have to look at it in a cost/benefit way. If you get your CS reduced to $50/month and the court case cost you $3k (DH's cost him $5k), It will take 5 years before the whole thing saves you a dime.
Sometimes you need to take a step back and not worry about what is right, but what will be best for you and your child.
Take care,
Ref