You would have to take the household income as a whole and divide whatever is left after payment of the household bills in the proportion in which you were bringing money in to the household.
You say that she has her own debt but very little income. Depending on how much debt she has if you were to put forward an IVA the creditors might feel that your income was going towards subsidising her debt repayments, therefore your own creditors are losing out. This might make them unhappy about accepting your IVA proposal, however, we'd need to know more about how much both of you are earning, how much debt she has, and how much the monthly bills come to.
Philip Beck -
www.freeivaadvice.co.uk
Licensed Insolvency Practitioner and IVA specialist