Originally Posted by KRP
You don't need this measurement accurate to four decimal places...or three...or two...which is good because you probably can't do that. .1" difference in sight height will cause an error much less than most people are capable of shooting. Pull the bolt back, measure firing pin to scope tube then add half the diameter of the tube. Or pull the bolt back and eyeball to the center of tube/rings.
I agree with KRP that .1 is "good enough" for this measurement, but also advise to not be sloppy about it. At closer ranges errors in this measurement can have a pretty minimal effect. However, as you reach on out there to 1000 yards or so the error can become enough to really matter. After all, with long range shooting it often comes down to a combination of small errors that add up to enough to cause a miss rather than 1 big thing.
Input all your data into a ballistic program, set it for 1000 yards then vary your sight height in increments of .1 and you will see the difference it will make. It may only be 1/2 moa at 1000, but that is 5".