I assume that you are attempting to prove that you are under the £5k trigger. So ignore what CF have done and challenge anyway. You will need a few valuations from things like estate agents listings, Rightmove etc. or one from a qualified valuer (which you will have to pay for). You will also need your current settlement figure from your lender.
Then look in your proposal to see if they stipulate the method of calculation --- as you will know from recent posts here, there are different methods emerging ! I would use the 'traditional' method in the hopes that your IP will accept it off the bat.
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