Hi Flowers2221 and welcome to the site. I hope you find the advice from all the posters to be of value.
The calculation of your monthly income is done by you - it is quite simple, just list out all your incomings - salary, benefits and other income, and then deduct all of your household and personal expenses, remembering to allow at least £50 for sundry expenditure, £30 each for car and house maintenance and approx £15 per person for medical expenses. (I often find people forget to include these items). An insolvency practitioner can then advise you as to whether your account is reasonable and likely to be accepted by your creditors. In general terms, and IVA is usually only viable if you have £250 or more spare money left each month, although you may be able to find a practitioner who will do an IVA for less.
The key is that you must be left with sufficient money to live, otherwise an IVA will not work. I can strongly recommend James Falla's book on IVAs and bankruptcy, which you can buy on-line from Amazon. It is written in layman's terms, and includes some very useful information on budgeting and there are some tables in the back that you can use as a guide for doing your calculations. Even I as a professional insolvency practitioner keep a copy on my desk and regularly use it as a reference point.
Once you have looked at your finances, I suggest you then seek professional advice to ensure that an IVA is the right option for you and your family. In the meantime continue to log on to the website, as it contains heaps of practical advice and experiences which will serve as an invaluable reference point.
Good luck
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
View my IVA blog at:
http://melaniegiles.blogs.iva.co.uk