AFAIK, the max tooth difference is about 14 and less is better. Try loosening the outer limit screw and increasing cable tension until, while in the big ring, you only have a couple of mm of clearance between chain and the inside of the cage while using the largest cog you would use in the big ring. On a DF bike, you might have to trim in the middle ring to access the innermost cog. With that large a difference between middle and inner rings, I'd install a chain stopper to prevent frame damage.