noflypaul,
Despite my friend JT's view, it gets down to what is reasonable in the circumstances, or the somewhat different animal "not unreasonable", at least in jurisdictions sourced in British law.
The judge may publish reasons that form a precedent ("ratio decidendi") or may make non-binding observations ("obiter dicta"), but they relate only to the statute as then applicable. Should there be a judicial interpretation that was unintended by the legislature (read regulator), there is normally a very rapid amendment to the controlling statute. Hence, there is much more structure and certainty than JT's post might otherwise infer.
Now, to your question...
Start off with the statutory requirements and any amplifying documents published by your State's regulator. If there is little in the public arena, there is sure to be internal records of the deliberations that led to the statute.
If all of that reveals nothing (the most common result) then you have to conjure up a risk assessment that is "defensibly conservative", ie has a level of conservatism that identifies and accounts for each risk without unnecessarily killing payload. A good example occurs when determining the take-off splay: if you have a choice between FAA (300 feet either side of centreline) or ICAO (90 metres either side expanding at 12.5% to 600/900 metres) or Australia (250 feet either side expanding at 12.5% until reaching LSALT), try playing with the arguments for and against.
As far as obstacles determined from a map or chart, then common practice is to establish the vertical and horizontal charting tolerance and then move the obstacle by the appropriate amount closer and higher to the take-off point. If you are dealing with a contour line, then you take the next highest contour level. Then you should apply a vegetation tolerance appropriate to the local trees.
And when all that hurts the payload too much, pay for a surveyor to get rid of the conservatism and get down to reality, at least close in.
Local environmental effects and aircraft handling are generally very difficult to cater for.
Have fun