Calculating Withholding With Graduate Tuition Benefit

In January this year, I began a graduate degree program at the same college where I work. The college offers a benefit of 90% tuition discount. Our costs are high, so the value of the benefit equates to ~$27k for the year.

 

I am afraid that this benefit is taxable and I'd like to know how much additional withholding I should be requesting to ensure that we are stuck with a big tax bill next year.