What is the correct Texas calculation for child support?

I read somewhere that to calculate child support for Texas- Federal, Social Security and Medicare taxes should be subtracted from your annual salary. When my child support was calculated they put me down as 145k total salary which at the time was really 125k with paid insurance and a car allowance. Is the car allowance and insurance considered part of my salary? With the deductions of taxes my salary comes 25k below the 100k mark for the maximum child support.