It's hard to know without seeing your pay stubs before and after (and don't post them, this is a public forum).
The issue may be how to get the withholding calculated correctly.
For example:
Gross pay $1000
Value of insurance (added to calculate correct withholding) +$175
Pre-tax deductions -$100
Equals taxable income for *for withholding purposes* of $1075.
Subtract withholding -$232
Subtract value of insurance (since it's not really part of your pay) -$175
Subtract after-tax deductions -$75
Net take home pay = $593
Is $300 the cost of your share? If so, then reducing your pay $400 would be $300 for the after-tax premium (your share) and $100 of taxes on their share.
If $300 is the value of their share of the premium, then your pay should only go down about $90 plus the cost of your share. It's really hard to know without seeing your before and after pay reports. You may need to keep climbing the ladder at HR, or maybe there is a friendly labor attorney who could review it quickly over coffee.
As a last resort you could file a complaint with your state labor relations board and let them investigate.
