Here is the law regarding garnishment
Maximum allowable garnishment
Except as provided in subsection (b) of this section and in section 1675 of this title, the maximum part of the aggregate disposable earnings of an individual for any work week which is subjected to garnishment may not exceed
(1) 25 per cent of disposable earnings for that week, or
(2) the amount by which disposable earnings for that week exceed thirty times the Federal minimum hourly wage prescribed by section 206(a)(1) of title 29 in effect at the time the earnings are payable, whichever is less.
In the case of earnings for any pay period other than a week, the Secretary of Labor shall by regulation prescribe a multiple of the Federal minimum hourly wage equivalent in effect to that set forth in paragraph (2).
If you have any other monthly payments such as child support or other collections due, this would lower you monthly discretionary income and may reduce the payment amount on the garnishment.
Title 15, Chapter 41, Subchapter II.