Child Support in Tex. How much?

I have one 6 yr old daughter, How much should I pay? I am currently paying $500 a month. My ex says I should pay more? I keep my little girl 10 to 13 days a month. I keep med ins. on her. I make $30 an hour. And we have not gone to court or anything the $500 was just what we agreed on when we split up.