My ex and I have a reasonable arrangement for maintenance payments for our dc. Never been through CSA/CMA.
His monthly payments go into a separate account to my current account and for a long time I just left it alone, hoping to let it save up for when ds gets older and there are big costs likes cars and university if he wants to go.
I always managed to cover the day to day costs by myself. Utilities, food etc. But as he's getting older he's eating a lot more and generally costing a lot more with socialising and activities. Now I find myself dipping into the maintenance savings a lot more. If he needs anything in particular - shoes, winter clothing, school transport I tend to dip into it. But then I feel bad because it feels like I'm not paying for any of these additional items. His dad is.
But then he lives with me 80% of the time so I suppose my food and day to day costs are more. Just interested in how other people work it out.