Voyager discussion

How is it determined at what date you will arrive in the past when you go through the stones? Is it always the same amount of years from your starting point, I wonder? And is it the same amount of years for everyone?
For example, in "11.22.63" by Stephen King (I HIGHLY recommend it, btw) the person going back in time will always arrive at the exact same place in the exact same time, no matter if he/she originally came from 1989 or 2011 or whenever.
But that's not the case in the Outlander series.
Claire first left in 1945 (ish?) and arrived in 1742 (? Sorry, it's been a while since I read it so I'm not entirely sure about the dates), so about 203 years back. And when she went back in time the second time, the same amount of time had passed in the past as in the present, so if Claire spent 20 years in the present, 20 years will have passed in the past as well, so Jamie will have aged the same amount of years as her, right? So for Claire, it'll always be a trip 203 (ish) years back, right?
Ok, now that we've established that, how did Geilis end up in 1742 like Claire (even earlier actually, because she came years before Claire)?
Geilis came from the 1960s (didn't she?), so she should've ended up in the 1760s, if the amount of years that you go back are the same for everyone, right?
So if Brianna goes back in time, there's no telling where she might end up, is there? It could be fewer years OR more.
It's entirely possible that this was explained at some point in the books, and I just didn't notice, so please don't jump at my throat if this is the case.

Thoughts, anyone?

I feel like the concept of time-travel in Outlander is more of a plot device to make Claire and Jamie meet. And in Voyager she travels back in the same amount of years only so that she and Jamie have aged the same and had the same n of years apart.
Time-travel seems much more 'natural' within Geilis' story, who was way less materialistic than Claire. In fact, the story would probably seem more coherent if Geilis was the one to meet Jamie. But then there wouldn't be that contrast of personalities that makes them C/J so appealing.
Also, the stones are turned on/off at convenient points in the story, which makes them even more of a plot device and drama generator.

So I guess you don't know what the determining factors for time travel are either.

Have read the other novels that come after Voyager...? A lot of your pondering regarding certain things are answered. :) I don't think I'm giving much away in this answer, but it may have a little bit of spoilers in it, but not much if so since the Geilis bit has already been addressed at this point I think.....

Geilis included human sacrifice which augmented her travel through the stones. Different gemstones also "boost (best way to describe, I guess) and protect" people when they go through the stones, which I believe Geilis also used. It seems like you can also somewhat direct travel through focusing on a person as well....