You would want to offer as near to what was due to be paid as possible. If there is a change of circumstances and your surplus has dropped creditors may accept less.
A settlement is usually based upon the number of payments left x the monthly amount. If there is equity to be addressed be prepared to add an extra 12 months payments in lieu.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014