There is no precise way to do this, because everything on a married joint return is calculated together. One solution is to prepare two married filing separate returns, figure out refunds based on that, and then apportion the actual refund based on that percentage. Or do the same for two single returns. Example: Married joint return has refund of $1400. Your MFS return has refund of $1200. His MFS return has refund of $100. You claim 12/13 of $1400; he claims 1/13.
