The middle number is expressed as a percentage of your width. So you use the width, the sidewall percentage and the actual rim to calculate the diameter in overall inches.
So a 245/45-17 is about a 25.6" height , which is stock size.
Your 315/35-17 is going to be a 25.6" also. So you won't need to adjust it.
This is what I use to calculate.
Tire Height Calculator