I hope I can explain this for you. If you still feel confused please don't be shy to ask questions again, I have no problem for questions
The ECO will eventually want to be sure that AFTER taking account of regular non-discretionary expenditure (such as rent or mortgage payments, council tax and secured loans etc) there is an amount remaining of not less than the income support levels.
This does not mean to say that you must save any specific amount each week/month etc.
It's also not an amount left over after all expenses. It's only the non-discretionary expenditure. (ie the expenditure that you have no choice on)
Spending on food for example is considered discretionary. You choose what to buy and how much to spend.
It only means that you must have that amount available to spend if you so wish.
Just for example if your husband is spending £111.45 on clothes and video games and not actually saving anything, he still has that amount of 'disposable income', just that he is spending it.
Hope this helps