Guys

This is a kind of random question. By optimising a website so it goes from page 12 on google to page 1 - is there an accepted industry average that says by what percentage the number of unique visitors will increase?

I know this is a how long is a piece of string kind of question but my 'superiors' want me to put a figure on this and, other than sticking my finger in the air and saying '50%' or some other random figure I thought I'd see if there is a rule of thumb that is generally accepted as being true. Ish.

I don't think me writing 'lots' will cut it really.