Rangefinder is not a Boolean perk; The further you aim down sights, the more benefit you receive

Edit: The title of this post should be renamed to "The further you aim down sights with shotguns, the greater effective range boost you receive".

This is most likely true for all weapons, but is most relevant to shotguns. This gif shows what I mean.

Here, I am firing my Party Crasher with Rangefinder at one of the portals in the Eye of a Gate Lord mission on Venus. I stand in the same place for all 4 shots. With each shot, I aim down the sights progressively further, until the 4th shot, where I am aiming down the sights fully. With each shot, you can see the damage numbers increase. This is because I am further zoomed in with each shot. This range increase due to ADS is true even for shotguns without rangefinder. Credit to u/Arkanian410 for pointing that out.

So if you use a shotgun, just know that it's always beneficial to be aiming as much down your sights as is possible when you pull the trigger.

As u/CptJero pointed out, rangefinder increases the zoom of your weapon. Since damage dropoff distance is heavily based on the zoom level of your weapon, it is especially important to ADS as far as possible with a rangefinder shotgun, as each frame in the ADS animation will correspond to a greater increase in zoom and therefore a greater increase in damage dropoff distance.

<Semi-useful advice>