I was taught to
a) Round UP the height to the nearest 100' then
b) Add 1,000' and
c) Add a further 300' for any unmarked obsacles.
For heights with a height recorded obstacle it was just a &b.
Well, there's three ways that you can determine the safety altitude.
First, you can scout out your route, 5 or 10 miles either side of track, for any obstacle or elevation figure, then apply what you said above.
Second, if you have a chart that is marked with Maximum Elevation Figures (like the CAA ICAO chart) you can add 1000 feet to that number in the "box"
Third, if you have a chart that is marked with Minimum Grid Area figures (like the Jeppesen VFR charts) you can simply use that number.
Looks like he used the first method from his numbers.
Obviously, you can always fly lower than the safety altitude, and sometimes you have to. You just have to keep a sharp lookout, that's all.