"For child support purposes, Net Resources = total earnings, minus Social Security taxes, income taxes for a single person, and dependent health insurance; calculator not for use if monthly net resources exceed $8,550 per Family Code § 154.125(a)"

Unfortunately since per diem is part of your total earnings, you can't exclude it. You can always negotiate an amount (Texas only issues child support *guidelines*), but good luck with that. Most exes go after the money at full speed.