I follow the vanilla code on unemployed settlers basically I roll to add a settler if:
1) there is at least 1 SS residential plot and 1 SS job plot available for assignment
2) there are no more then 4 unemployed settlers. (settler unemployment is actually returned by the workshop itself, so I cannot confirm the method by which unemployment is tracked).
Then, if a settler was added I: check if the brahmin setting is enabled, check that the settler isn't a guard preference settler, if they are not and there are less then 1 Brahmen per 10 settlers, then attempt to attract a Brahman to the settlement as well.
That's basically it. I don't compare "settler count vs plot count" because some people play a mixed game of SS + standard items, and such a system would screw them over pretty hard, so instead I make it run on unassigned plots. (this allows the enterprising player to leave a home/job unmanned to attract any number of settlers until their game melts).
Having said that, I'm certain my beacon is attracting more than it should (I have several independent reports of it), however what I don't understand is how (I've never been able to replicate it).
My sneaking suspicious is that it has to do with a bug in how the game reports the vanilla settler count/unemployed numbers, but I can't confirm that. The Pip-Boy numbers bug suggests that there is a problem with this reporting, but I don't know if said issue is purely cosmetic or exists on the actual reported figures as well. When I rebuild the system, I'm considering making a max number of settlers that can be recruited between settlement visits/cell loads to stop the issue from spiraling out of control at least.
The other possibility is that settlers aren't being assigned to a plot when they are attracted but a) I can't reproduce that either, they always get assigned for me and b) even if that were true you should end up with at most 4 unemployed settlers unless they get assigned to vanilla objects.
The only case I can think of that should potentially attract 2 settlers (after the initial beacon startup) is if you have multiple beacons in the same settlement (I'll be fixing that, it's a known bug), but even then, you should get at most N-1 extra settlers where N is the number of dispatchers (in the same settlement) if all the settlement numbers report correctly.