Hi,
A Mosfet is no ideal ON/OFF switch.
It has an analog region between ON and OFF.
This analog region will drift:
* from part to part
* with temperature
* ... and it depends on load
And V_GSth tells you at which level the mosfet starts to get conductive, mind the tiny drain current.
If you want it to hard switch between ON and OFF you need some extra circuit.
Some kind of amplifier with positive feedback (hysteresis).
You may use a comparator IC for this (more precise), or you may use an additional transistor ( more simple, more difficult to calculate exact level, drifts with temperature).
If you want to control the 11V limit more accurate than you need a reference voltage.
To prevent it from "chattering" (ON OFF ON OFF ...) you need to consider a hysteresis. This depends on the source impedance.
The problem is, that when you switch ON the load then the input voltage may drop a little and it will become below 11V .... causing to switch OFF .... then the voltage will rise above 11V again ... and it will switch ON .... repeating..
A hysteresis makes to switch ON at a given input voltage. Lets say 11.0V ... but then it will stay ON until the input drops to a lower value, let's say 10.5V.
I've given some options ... so it's in you which way to go.
You need to decide how accurate you want it to operate.
..how exact (which range) the ON threshold should be
.. and the OFF threshold range.
Klaus