The current regulation (EU) 2021/535 establishes that the device must be inserted 

into the mudguard complying the following rules:

A) The part “A-D” of the mud-flap device that is inserted into the mudguard must cover at least the tread of the tire or tires in the case of twin wheels (Fig. 1 and 3) and cover an arc of at least 30° from the horizontal axis of the wheel.

B) The part protruding from the mudguard “B – C” must cover at least the entire width of the tire or tires 

     (fig. 1 and 3) up to a maximum of 200 mm from the ground and 300 mm from the tire (fig. 2).

Here some of the most common couplings between mud-flap and mudguard