I think you'll struggle to find a solid suggestion since disposable income isn't something people directly budget for; it's what's left over after you've ensured everything else is budgeted for.

The thing is, what is your disposable income for? Going out to the movies every weekend? If you enjoy movies that much, why not roll it out of 'disposable income' and into your regular monthly budget?

Or did you have a more refined definition of disposable income?