You’ll notice that the G45 has eye relief, a distance at which you can perfectly see the edges of the inner tube.
If you start with the G45 say a foot from you eye and slowly get closer you’ll notice that you’ll start with a really bad view through the scope. As you get closer there will be a distance at which you will see all of the possible surface area of the optic. That’s your distance. Keep in mind it’s best to have the rifle shouldered in the correct position.
Once your have that distance you can put the EXPS where you’d like in front of that.