For those of you with older kids, how much flexibility is built into your visitation schedule?

We've only been separated for a couple months, and for the second time we're running into a conflict. The first time it was the stomach flu in our house, and now it's a sporting event. DH didn't want to catch the stomach flu, and he does want his participating in sports, so he's not protesting, but I'm wondering if I should be documenting when we change visitation, or if we need to rewrite the agreement to explain what happens in these situations. Or if it's Ok if we just wing a solution.

Right now, our parenting agreement gives me full custody, and him one visit a week supervised by a member of his family, so missing that one visit is kind of a big deal.