A F&F is based upon the remaining payments due, plus 12 if there is likely to be equity release. It can be rounded down a little, depending on the time left and the amount to be repaid by the IVA against the total debt --- the higher the return and time saved, the more likely the creditors will be content to lose a little more. It is a very imprecise science and I have seen high offers refused as well as low ones accepted.
I might also add that the circumstances have a bearing as well.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk