Your right Troopietraveller, i did only want a ball park figure, and even then i know it could be wrong. And Noels comments are interesting, that there could be some maths used at the top to work out whats needed? But what ever anybody says, i have got a lot of usefull information from most people writing on this thread (like smudgers link), but what i will do is go through all of my books,and see if i can find the % ref. (Would it still be said it cant be worked out, if the person writing the book is very high up in the forces? i wonder?) thanks everyone adam