If the base of the case is flush with the gauge you are at maximum SAAMI headspace. There is a notch cut into the gauge so that you can see minimum SAAMI headspace.
If the head is sitting proud of the gauge the die needs to be brought down a bit. If the head sits below the minimum headspace notch the die needs to be brought up some.
On the other end which is shown in the picture you can see the maximum case length and the notch for minimum case length.
I generally use this for my semi-auto rifles. If it fits in the case gauge it fits in the chamber.
For my precision rifle I use a headspace gauge to measure a fired case and set my full length die 0.001" shorter.