marco 888;
Hmm, yeah it does more or less help, at least I understand it a bit better now. The reason resides on the fact of it being inversely proportional and therefore the rule of three is not applicable. It's a bit of a mess anyway coz in order to what you are asked for, you apply one method of calculation or other, there's no one straight way of doing it. It's relative.
Thanks for the explanation.