OP do you make substantially more than him? if so, this could affect what you get from the modification.
Here are the basics for Maryland:
Maryland Child Support Guidelines
The guidelines include a schedule of basic child support obligations that shows the total amount of support both parents must pay based on their combined “adjusted actual incomes” (explained in detail below), and the number of children they have together. A percentage of the total support obligation is assigned to each parent based on that parent’s income percentage. For example, if parent A earns $6,000 per month and parent B earns $4,000 per month, parent A would be responsible for 60% of the support amount (6,000 divided by 10,000) and parent B for 40% of the support amount (4,000 divided by 10,000).