In a Practice Forced Landing (PFL), what are Low Key and High Key and how are their locations determined?
Practice (or real) forced landings and (SFO) simulated flame out (or actual flame out - engine failure)  maneuvers are done often by the military in the fashion shown in the pictures below. Part of the reason for this type of maneuver is due to the characteristics of the aircraft involved.  For example, high speed fighter jet aircraft need to fly fast and they use this maneuver to dissipate speed and altitude in a relatively short space. 

Regarding the SFO or actual Flame Out:

The High Key  position is generally when the aircraft is somewhere over the first third of the runway and is the point at which the pilot will begin his right/left turn and start his descent (if currently level). If it's an actual flame out, then the aircraft may be in a descent prior to reaching High Key but should be attempting to be at or near the altitude that was practiced during a simulated flame out (when training).

The Low Key position is generally abeam the approach end of the runway.

These "Key" positions serve as altitude and configuration reference points in order to ensure that the aircraft will be at the proper position, speed, and altitude over the end of the runway. 



The altitudes (which may vary based on aircraft type) utilized for the SFO and forced landing maneuvers are based on above ground level (AGL) and the AGL altitudes won't change based on runway elevation.



The pictures below show how these maneuvers generally are done:

From: laartcc.org/stm/special-military-procedures