Normally you use the amount of payments you have left and offer those. Don't call it a settlement, just a full and final offer. Where would the money be coming from?
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
As Kallis says --- a full and final amount is based on the number of payments remaining (plus 12 if there is equity to be addressed) multiplied by the current monthly payment. Depending on the time saved this could be rounded down a little. (A settlement figure, as Kallis alludes, is a different thing, being the balance of the full original debt, plus fees and possible statutory interest ).
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014