What were the details of your test?  Were the kanraks fully upgraded?  Were they VL or VR upgrades?

With all 6 culture techs, culture is a swing of 6% in shield mitigation...furthermore, PMs get at most a 30% chance to bypass shields, right?  So the culture techs are also subtracting 20% from that, leaving only a 10% chance to bypass shields...6% extra shield mitigation would decrease incoming damage by 1/5 (20%) if you were a cap at ~70% base max shield mitigation...compounding the 10% chance to bypass shields, 13% increase in survival time seems reasonable...

My biggest question is: is the culture buff multiplicative or additive with the PM bypass chance?  I assumed additive, but if it is multiplicative then the 20% culture buff would be 20% of 30%, which is only 6%...that'd still leave a 24% chance to still bypass, and if that's the case then I definitely agree that is a pathetic buff...

Should be pretty easy to test: set the first culture upgrade to perhaps 50% or so and see if any phase missiles are getting past shields.

The next step if it were shown to be multiplicative would be to try setting one of the culture techs to 100% PMB and see whether it negates phase missile bypass entirely or simply divides the bypass chance by 2.

[EDIT: While I haven't yet done any testing the more I think about it the more I'm coming to believe the advent techs are multiplicative. Think about it- they add to an existing entity modifier(ChanceToIgnoreShieldsAsDamageTarget) And pretty much all entity modifier buffs act multiplicatively on some existing stat.  Not to mention one of the only other two effects to use that entity modifier(Adaptive Forceshield) At rank 4 goes all the way up to 75%- Being that only 30% phase missile bypass chance can be researched or 50% IF shield disruption were additive, why would The devs even both increasing the phase missile block chance at rank 4 further in rebellion(in trinity it capped out at 65%).

Those would be some odd inconsistencies if "ChanceToIgnoreShieldsAsDamageTarget" were in fact additive.  Moreover, it seems abundently likely that this entity multipliar handles reductions the way all other reductions do in sin.  Which would mean 20% phase missile block actually reduces 30% shield bypass chance down by 1/6 (1.2^-1=5/6)- which is to say to 25%.  Which is to say each 5% phase missile block actually negates about 1.25% shield bypass chance.]

"I am tired of loosing level 10 capships to a few bombers all the time"

do what i do, add flak burst to every capital ship that does not have point defense be it tec or not