That unknown distance range sounds like it would have been fun to try >
The "Open gate" method is excellent for shooting teams. I don't suggest it be used otherwise. It is a method that allows the shooter to correct for the pre-calibrated elevation drum not being perfectly matched to individual ammo/rifle combinations.
I will harken back to the days of the Iron sighted C-7 for a moment.
Back in those days, there was considerable effort and thought given to how best to setup the non-adjustable sights for varying engagement ranges. The "best" compromise seemed to be a 400 meter zero.
This resulted in the following elevations:
100m 7" high
200m 11" high
300m 9" high
400m 0
500m 18" low
Relating this to targetry, between 100 and 300m, aiming at the bottom of a figure 12 would drop the rounds into the face (between 7 and 11" above your aiming point). That means that for most of the shooting you do, your MPI will be centered within the same 4" vertical area.
At 400m, your rounds hit where you aim.
At 500m, cover the figure 12, and your bullets will drop into it (ok, into the bottom of it, the Fig 12 is only 22" tall, but if you frame the front sight on the 4 foot screen, it works well.)
How does that relate to the Elcan? Well, the first year I shot an Elcan, I zeroed at 400m and didn't adjust the elevation drum at all. Worked out alright. I later learned to use the Open gate method.
This technique may also be known as a 6'o'clock hold zero.
YMMV, but there may be some value in a history lesson.
NS