I may have misunderstood the proposed rules, but I thought it was: point defense damage / 4 = capital damage value of missle => 50% chance of interception

So to intercept a Barracuda (2 capital damage) you need a point defense bay with a damage value of 8 or higher. That's 8MG or 6 Small Laser (damage value is halved for point defense work for everything but dedicated AMS). I don't know how it works for weapon bays that feature multiple öaunchers, whether the Point defense bay gets a chance to fire at each missile in the volley (for additional heat or ammo) or only once?

But regardless, missile interception chance is at most 50%, so missile proofing is under house rules not as easy as under RAW.