Hi Mario ,
For achieving a good result on the moment you have to give punch on the throttle ,both pedals(brake / throttle) have to be on the same position.
20 mm seems to be a good value, but you have to choose the thickness which one you feel great doing this .
Just make tests with differents values with woddens blocks which cost anything.
When you find the right value because you fill good when driving you can use a another mateial ( alu etc..) or you can keep it in wood.
@+