Thanks Barit1 .... the 2.5 to 3 factor is a starting point for a rough and ready comparison.
Since my original post I`ve been doing a bit more reading and find this topic has been discussed quite extensively on the Web in a variety of locations. The general view being that you can`t compare apples with pears.
For the time being I`m going to confine myself to comparing like with like but give a bit more thought as to whether I can come up with some qualified notional (rather than scientfic) comparison for illustrative purposes. So any more thoughts along those lines on the subject would be welcome.