Yes technically that is true. You could also add the additional amount paid in 2011 (based on the January 2012 paycheck) and then divide by 12 for the monthly gross amount.
I am going to disagree with your approach. Your ex husband was alive for 12 months in 2011. He had income during that time period. What he received in 2011 is divided by 12 to get the monthly gross. I realize this makes for a lower gross than if you divide by 11.5 months, but the child support guidelines are based on actual income received, not earned.
My question to you is: if a paycheck is dated 12/17/11, wouldn't the next paycheck 2 weeks later be 12/31/11?
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