I would be of the opinion a flare will cover up to a possible 50mm protrusion, there is a legal width limitation i do believe? From real world experience no flare equals ship loads of mud flung in all directions from tyres that barely stuck out by 15mm, brush rails did diddly squat all !! No mudflaps = 110$ fine, an flaps must be a maximum of 300mm from the ground < it's all about slinging stuff at other road users, i don't see guard protrusion as being different? Then the track width changes are regulated to a 50mm increase as well?
It's only illegal when and then it's too late.
I've not heard of any one with buntings plastic garden edge flares being pinged for it, yet? < they look ace as well