The amount should be based on the number of remaining payments multiplied by the monthly amount and could be rounded down a little if there is sufficient time saved to allow the creditors to make savings on administration costs.
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