Exactly what are the limitations of geofenced reminders?
I'll split your question into two main areas since one is easily answerable, the other isn't without inside knowledge of how location services work within iOS.


How to make a reminder for when you re-arrive at the place you sit now?
What is the special sauce and all the ways I can tweak location in iOS?


To make a reminder, start by turning on "At a Location" and then choose when I arrive instead of the default when I leave. Sometimes I have poor location signal and the alarm may go off, but that seems to happen very rarely in practice. When that happens, I just set it to when you leave. That seems to be less sensitive to radius so when it goes off, I then remember to change it back to arrival.



The deeper question of how location really works and is implemented is not publicly disclosed. As an end user, you can't set radius or other parameters that affect the location engine. It is either off or on. You either have the radios on or off. 

I would encourage you to get Xcode and play with some of the demonstration projects to learn how location works in the iOS simulator and perhaps you can gain a better understanding of how iOS presents this data to apps.