Do not let him get away with 50% as you don't want to set a precedent.
Depending on your stbx and your relationship with him I'd either send him an e-mail:
I saw that you dropped off a check for $50. The laws in our state are clear that, because you make 60% of the income, you must pay for 60% of the add-ons. That means you owe $60 for June; you are $10 short. Please pay me the difference by June 25.
If I don't see the money by then, I'll have my lawyer contact yours, but I'd rather not have both of us incur that additional expense.
OR, if it's going to upset you too much to have this conversation with him, just go directly to your lawyer, though it will cost you, especially if you think he might comply on his own.
Good luck! Don't let him get away with it, though!!!Me: BW, divorced, now fabulous and happy!
Married: 11 years, no kids
Character is destiny