Looks a lot like my Porter. I use 20% Munich, 7% C60, 5% each of carapils, chocolate malt and roasted barley, and the rest is MO. I shoot for about 1.047/4% ABV, so I mash at 160 to leave a lot of body.
I think 25% Munich will be fine. You could also consider using a different base malt like MO to get some more malt flavor in there as well.