Splitting college expenses in SA

I would like to work into the Separation Agreement that my attorney is drafting an item(s) about splitting the children’s college costs (they’re currently 11 and 12). However, based upon our current situations, it’s likely that neither of us will be able to afford 50% of the tuition, even excluding anticipated financial aid and (hopeful) student contribution–or if a state college is used as a ceiling. Is there a good way of wording it (income-dependent, based on ability to pay?) to allow for this probability? And how could each parent’s ability to pay be gauged? I don’t want to fall victim to language that’s too vague to be enforceable.

I never advise my clients to agree to pay for college, the court cannot order it, and you never know what your financial situation will be in the future.

If you must bind yourselves to this uncertain future expense, perhaps a safe way to word the agreement would be to say that you will contribute X% of your gross monthly income to the child’s college expense.