what is the % health costs need to be compared to total gross income to write them off?

Answer 1

You'll be able to deduct the amount of your hc costs that exceed 7.5% of your AGI. This gets added to the other deductable items on your schedule A. So, depending upon what other itemizable deductions you have, and the standard deduction for your filing status, the hc costs may or may not end up reducing your taxable income.

There are a lot of things classified as a health care cost that you might not think of. Mileage to the doctor, long term care insurance premiums, medically necessitated modifications to your home, ... even expenses for someone who might not otherwise qualify as your dependent because of their income. Be sure to read up on the deduction in the instructions for Schedule A, or consult a tax professional.

Answer 2

7.5% of your AGI, which for many people but not all is the same as your gross income. Medical expenses over that amount are eligible itemized deductions. You would only itemize if your total itemized deductions is higher than your standard deduction.