Looking back over my last post I think it is easier to conceptualise by using a new stat "clicks to convert".
So lets take a practical example I have a client who has a keyword that has 567 clicks with 9 conversions. so:-
567 / 9 = 63
So every 63 clicks will result in one conversion, lets say the target is £5 CPA so:-
5.00 / 63 = 0.079
So in this case the max bid to achieve a £5 CPA would be £0.08!