Hi john Many thanks for the quick reply, and clarification. I now understand how this calculation can be done. Please note i have viewed all your lectures in relation the the f5 paper, actually, i revisited the lecture in relation to the doubling rule this morning before i wrote to you. But there is nothing in respect to the reverse calculation as in this example. However, please accept my apologies if i have missed anything in the lecture which should have avoided me asking the question. Lectures are really good and very easy to follow. Many thanks emerson