Is this a troll?
Above the transition layer you use 1013, below it you use the RPS for FL/altitude reporting. Simple really. The only variable is the altitude of the TL itself. And the RPS of course.
The quadrantal rule (or semi-circular rule) is formulated so that it uses altitudes (thus based on the RPS) below the TL, and uses FLs (thus based on 1013) above. Same goes for the vertical limitations of CAS.
At 5000' I doubt you'll hit any terra firma
I'm searching for an example where there is terrain that extends through the TA into the Flight Levels, and which would make a re-calculation of that terrain altitude to its corresponding FL necessary to stay safe. Does anyone know of such an example?