Child visitation/support!

For all those who know or have been through this...I'm going to court this Thursday for child support/ visitation rights. Their father tells me their only going to give me $500 if that per month. I honestly don't care what they give me....his sorry ass hasn't supported his kids since day one! But I'm curious to know I looked on the internet and it said for two kids he'd have to pay 25%. Well I know he makes anywhere between $3000-$3600 a month. With those numbers he'd at least owe $650-700. Am I right or totally wrong???