When I did my skills test, the examiner did expect me to calculate a saftey altitude. He asked how I calculated it. I havd calculated it just as IO540 described above
I think the answer that is expected from a PPL is that one draws the track line on the chart, and looks 5nm either side of it, take the highest obstacle found, add 1000ft to it, round the answer up to the next 100ft, and that becomes the MSA for that entire leg of the route.
The examiner commented that he was delighted, because he wouldn't have found the MEL (from the grid on the chart) +1000 to be acceptable.
dp