I think that sums it all up…
I am all about self experimentation and owning up your own risks.
But in order for someone to be able to responsibly accept the risks they are about to take, they must first be able to understand those risks.
And I think this is where things went awry on this thread:
Absolutely no one who knows what is nerve damage would ever utter that sentence.
A (very) wrong pinch on your fingertip nerve might cause you to feel debilitating pain whenever you bend your elbow, for the rest of your life.
One thing is to go in for a procedure knowing that you might come out with bad consequences. This will allow you to be extra careful and prepare for any eventualities.
Another thing is to just ignore the risks blindly.
Being aware of what might go wrong allows you to prepare well enough so you can turn failure into success.
Another big red flag here was this:
Fisrt off… Don’t think that black market Special K will be even nearly close to actual Ketamine.
1 vial of medical grade Ketamine becomes up to 30 vials of ketamine before it’s mixed up with other shit prior to hitting the black market. (why else you think it’s so cheap?) 
Not to mention the most important: By altering your mind before operating in yourself you are just inviting disaster.
Have you ever considered what happens when your blood pressure suffers a sudden drop while you’re under the effects of Ketamine? You drop uncounscious.
So how close are we from the scenario where you start poking the wound with the needle… and suddenly falls to the ground (with a sharp object inside you)… when you come back to your senses a few hours later you have a silly wound that takes a month or two to heal before you can try again and your magnet is no longer sterile.
That is a great example of tragedy that can be mitigated by prior study.
Also, Despite being in favour of self-experimentation, I do agree strongly with @Coma:
And sometimes all it takes is one bad fuck up that falls in the media to ruin the practice for everyone.
Take UK for example… Currently it is the worst country in the western world to get bodymods done. All the studios stopped doing interesting stuff and most places won’t even do basic experimental stuff now. All because of one mod gone “wrong” last year that hit the media.
Now just imagine if they had been using Special K on Dr Evil’s procedure. 
…
Well, now that this is out, let’s get down to business:
I would rather avoid getting any needle through the Phalanx joints. Far too much risk for messing up your mobility that can be mitigated easily.
@mrln’s approach and placement seems ok. there are just 2 elements to consider there…
First is xG3’s diameter. 3mm is a lot for a fingertip.
so…
This
is a 3mm space. it is as close to the sides as
reccomended.
And this
is a rough represendation of the wdth taken by bones and essential icky bits which cannot share a space with a magnet.
There you can see a rather undesired overlap…
This does not mean it’s not possible. Quite the opposite: that place is canonical for magnets. but what happens is that the magnet pushes the flesh out and the bits in.
So the maximum diameter we can insert there depends a lot on your constitution.
Unfortunately just like not everyone can accommodate an industrial piercing, it’s even harder to fit a 3mm x15mm magnet on the fingertips.
Although by your pictures, you might be able to fit it in there… but be prepared for possible rejection.
Now for the second issue… the length.
Many people only take the length of the fingertip into consideration, measuring like this:
That works fine for a lateral placement, but you must take into consideration that whenever we grip our flesh changes in ways that a glass cylinder cannot.
So the closer we are to the centre of the fingertip, the smaller is the available length.
These
Diagonal lines should work as rough safety guidelines. so you might play with placement by following them instead of just moving the measure or 3d model in a straight line.
Sort of like this 
That can also guide you into how close to the centre of the fingertip you can safely place it. ( not that anyone wants a magnet on the middle of the fingertip )
In short, it is of course possible to place an xG3 in the fingertip, but that’ll depend a lot on your anatomy. It might just as well fit in perfectly, get rejected, or just be very visible.
Another potential placement candidate might be the position “-1”, or just outside the metacarpal bone for your thumb…