Calculating Child Support Owed Idaho State

I am paying $1,200 per month for 2 girls in Idaho State. I made $29,000 in 2009. I have to supply insurance for both which costs me an additional $100 per month. Our custody agreement states I have the girls every-other month but I seem to have them almost every week end.

The mother of the girls says i must pay her gas to bring the girls to where I live or to pick them up. She tells me she is being generous and could get more support. How can I find out what I should be paying and what rights I have?