Can you right off or claim child support ?

How is child support taken out of your check percentage or set amount weekly

You have to file for child support modification.

The only way you can write it off is to have it changed from child support to "family support". Only then does it become tax deductable. But most ex's don't want to pay the taxes on your money so its most likely not going to happen. Spousal support is deductable.

After there is a child support order, if your income decreases you can file in court to have the amount changed.

How it is figured for ( how much) varies in the US by state.
A rough figure in GA is about 25 percent of your gross pay, if you have just min visits with child.