Belly adjustment in the All Day Shirt

I'm making an All Day Shirt for a friend for his wedding. He's a fairly big guy. I made a muslin in the XXL. Overall, I feel like the fit is pretty good-i like where the shoulder seams land, the sleeves need to be shortened slightly, but that's an easy fix. It definitely meets, and I like the ease in the chest, but there is less ease at the belly. I'm think just 1" total at the belly would be helpful. Can this be done as simply as grading out .25" at the belly on the fronts and back, or do I really need to do something more technical?