Usually an offer would be based upon the number of payments left x the monthly amount, so the creditors are getting about what they expected. If you have equity to be addressed then it is usual to add on 12 payments in lieu.
Any exceptional circumstances causing the need for an offer ( losing income, health etc) can be used to help the arguement when making a lower offer.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014