As you can see in the table, the difference between wide angle - placing the projector as close to a given sized screen as the lens will allow, and the mid-point on the zoom only causes a modest 10% and change drop in brightness.
That's actually very good. That is, the drop of 10% is modest for a projector/lens with a nice long 1.8:1 zoom ratio.
As expected, (as it seems to be typical), the drop from mid-point on the zoom to full telephoto (placing the projector at the back of its range) is significantly greater.
Going from closest positioning (wide-angle/zoom in) to furthest placement (telephoto/zoom out) yields a reduction of 36% in brightness. That's pretty good for a 1.8:1 zoom lens.
If you want maximum brightness, mount or place the projector near the closest possible positioning. It is true, though, that at the short end of the range (wide-angle) there's going to be a bit more optical distortion (such as barreling). Also, if you are working with screens that have significant gain, and a narrow viewing cone, the audience will see more roll off in brightness in the corners and sides, compared to the impage center, due to the steeper angles between lens and those edges. So, closer placement does narrow the ideal viewing cone.
Fortunately, if you need to mount around the mid-point, the loss in brightness is minor. At the full telephoto end of the range, brightness is down (about the same amount as switching to Eco mode), which is significant, but not drastic. Of course, this Epson has a lot of horsepower, so that it shouldn't be much of an issue.