Hi - I paid my IVA 20 months early but I had to pay in full what I had remaining. I was told that would guarantee my offer being accepted by the creditors, I could have offered less but I didn't want to take the chance of my offer being turned down.
Starting point is usually remaining monthly payments x current monthly amount. Creditors will entertain less according to circumstances .. illness, loss of income etc. Don't forget to factor in any equity they might be expecting.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014