Defensive bonus defined by hull space and speed ?

Hi, i am quite new here so i guess its ok to ask some noob questions.

As the topic title already suggests, i dislike the static bonus and tried to mod the defensive_to_hit bonus for my ships depending on the hull weight and the speed of the respective Ships.

I tried two different ways, however both didnt work.

The first attempt was to mod the hulls directly, got no errors in the formula but it didnt work as expected. it seems that the values are oly checked once during the hull creation and not dynamical while you add components.

The second attempt was to give the ships a 0 def bonus in the hull and mod the bridge/Computer components with the Get_Design_Ability_Total "Movement Standard" and "Movement Bonus" function.
Cant remember the formula but it was about the same result, no errors but didnt work.

Any ideas, is it possible at all ??

2x