It comes pretty close, especially for the lower elevations, so you could use it as a check to verify if your result obtained from the chart is correct. See for yourself if it works for Cochabamba airport in Bolivia (elevation 8360) and QNH 970.
On the other hand you are 4 hPa off in your example, and according to your 30 ft / hPa rule that is 120 ft too low. I am not sure if you'd be comfortable with those kind of errors when it comes to say... decision height?
Good luck on the tests!