It is just a yes or no question, you cannot put in a value.
The IRS defines legal blindness as being unable to see better than 20/200 in your better eye even with corrective eyeglasses or contacts, or having a field of vision less than 20 degrees.
Be sure to get a statement from your eye doctor or registered optometrist verifying the condition of your eyes, and keep it for your records in case the IRS requests it.
Please check out this TurboTax article for more information: https://turbotax.intuit.com/tax-tips/disability/tax-tips-for-the-blind/L80haiutG
