You're flying in the UK right? Well Glueball is basically right that you use the MEA/MSA on the chart. If you're using your Half Mil then take the figure quoted in your square, add 300' because that is the max height of a structure in the UK without informing the authorities, then add a further 1000' for safety margin and you have your en-route MSA. If you want to be more accurate then you can do the same sums for the tallest structure within 10nm either side of your track.
Within 25nm of an aerodrome you can use the MSA quoted on your Aerad/Jeppy plate for IFR procedures directly from the plate using the numbers given directly as the safety factors are already accounted for.
Hope that helps...
