@yjsk2100,
I've no idea, but maybe the rules for employer garnishment of wages might be a guideline. *Fifty percent is the most an employer may deduct regardless of the amount ordered and regardless of how many such garnishments are in effect. Again, I've no idea of the rules for prisoners.
*Some states required up to 60% if the account is seriously in arrears. Certainly, if he's in jail for not support, he is seriously in arrears.