You have to base it on the increased payment, so 42 x 154 ..... £6464. CF are notorious for grabbing as much as I can and will probably say you should offer far more, but, with any other firm I would have said around £5k would be a good offer.
I am sure that you have read the stories about CF, so I will not repeat them here, but stick to your guns. These things are for the creditors to decide, not the IP.
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