Basically they use your take home pay and deduct agreed, allowable expenses, per accepted guidelines. What is left is your regular IVA payment. After that they also take around half of any susequent pay rises as well as overtime and bonus payments and all of 'windfalls' over £500 ( lottery wins, inheritances and the like).
You have already been through this exercise and the details will be in your paperwork.
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