I have a couple of guitars that I need to adjust, and I'd love to be precise with measured numbers. So this and the feeler gauges will go very well together to get the neck relief where you want it.
But I will not recommend this for professional use, since it's not precisely made. I happen to have a carpenters level laying around and the 24,75" is not perfectly straight, but the 25,5" is. So the 24,75" won't give you precise straight reading of your neck. Also when I look down at the length of the straight edge closely, I noticed the straight edge has a little curve.
It's nice for home use for your own guitars, if the super precise adjustments are not your goals.