The tax change implements a new 10% bracket for the first $6000 for single filers, first $10,000 for head of household, and $12,000 for joint filers. That bracket is currently 15%. The difference between the 15% and the 10% is being refunded retroactive to the beginning of this year.
Single filers get $300, head-of-household gets $500, and joint get $600. The only way to get less than those amounts is if your taxable income for last year wasn't as much as the above income levels, and then you get proportionally less.