It really does help to have that buffer of a lawyer in the mix, too.
What I did, when my last ex sent a letter whining about how much money he got out of the sale of property we owned and demanding more, was to write a letter spelling out what he had agreed to, pointing out that once the capital gains taxes were paid (by me) he actually owed ME money, which I would forgo unless he made an issue out of how much he got. I emailed it to my lawyer, who put it on her letterhead and sent it. Never heard a peep after that, and I think my legal fees were like $50 for the time spent reading my email and putting it on letterhead/mailing it.
Good job--and this will get easier, with practice.
ETA, the money he would have owed me was like a thousand bucks or so, but this was a $300k-plus property sale and I had no problem absorbing the extra taxes for the sake of being free of his whining. But I wasn't about to give him a penny more.