Generally speaking the high levels of cover are things like bunkers and bulkheads. So assuming a 3 foot by 4 foot shield (about 90cm x 120cm) with a faily conservative foot thick (about 30cm) and mild steel being 7.85g per cm^3. That comes to 324,000 cm^3 and 2543kg in weight, or about 2.5 tonnes.

"mild steel is 7.85g per cm³"... ok I don't know about that but I believe you and it's ok for me too agree upon "plasteel probably is about as dense as mild steel"

so a 90cm*120cm shield with a weight of 9kg should be about 9500/7.85/(90*120) => 0.11205473 cm thick (if my formula is right) and those 0.112 cm provide 8 AP

a guardshield with the same material weighting only 1,5kg should be (1 500 / 7.85) / (90 * 120) = 0.0176928521 thich thus 0.018 cm provide 6 AP

if we try to create a function x(thickness)=AP with x(0)=0, x(0.018)=6, x(0.112)=8 should it be a logarithmic function? should it be an exponential function? and what would be the required thickness for 16 or 32 AP ?

I think we can see that either my formula is wrong, or the density is somewhat wrong or it is just really ridiculus to approach it this way...

I agree that the naval shield should not count as "total concealment" but I think it shows that fluff text is just not enough for shield rules

and about flexible weapons yes they can't be parried but that's not a problem of shields, shields are not superior or inferior to anything else here

but another question... what does craftsmanship do to shields? as it clearly is a weapon it does not receive the plus 1 armour and as I don't get the +5/+10 for blocking... what's it good for? (except for the mono acreage buckler which is a ranged weapon if thrown and therefore does never jam ;P )

A milimeter of mild steel at about shield size 2540kg / 300 = 8.43kg. What's a Naval Sheild weigh, 9kg? Seems about right to me. Maybe that guard shields a bit light, maybe plasteel is nothing like the density of mild steel.

The deescription of cover isn't exactly very precise in the main rules. But seeing as 32 points is more than a Guncutters frontal armour (30), far more than a chimera's (27), over twice the highest armour rating of ork Mega armour (14) and described in the Apocrpha as Heavily armoured vehicles. I mean that chimera's thickest armour is say 150mm thick (http://wh40k.lexicanum.com/wiki/Chimera) (1.2 tonnes of mild steel) i'd say it's pretty immobile for most part.

well with the correct mecha a tech-priest can lift half a rhino (but probably don't move it around) and ... what about 16 AP? 1000kg is carryable for a techpriest as far as I remember

well carrying capacity stops at SB+TB 20 as an ascended character with unnatural toughnes and strength you should be able to push for about 30 without serious trouble

TB+SB 18 is enough to carry up to 1.35tonnes with ease

and the table stops at TB+SB 20 but 2.5 tonnes as Face Eater was talking about is absolutely doable at ascencion level

in the corebook on page 368 there are metal crates listed for a 32 cover... plasteel should have more AP/kg as I think if not I can carry around an empty metal crate to fulfill my needs

but as rules on page 199 are (table 7-10 cover types) heavy wood has about the same AP as my naval shield and as I don't want some "shield" to carry around but some real "cover" a stone statuette should already have 16 AP (or a plate that should seal of a crypt on a graveyeard) now what about my steel front door?

now to come back to armour plating... a rhino armour is about 30-60mm (but I don't know about armour values) while a chimera hast 100mm

rhino has 21, chimera has 27 AP (the io pattern termite has 35 in it's front)

even if taken 60mm for granted 40% less in thicknes is nowhere near the 22.22% less AP so it seems to be a (square?) root-function meaning I have to add only little to get from 1AP to 2AP but much to get from 20AP to 21AP

(so now our little function looks like this:
x(AP)=thickness in mm with
x(0)=0; x(6)=0.18; x(8)=1.12; x(21)=60; x(27)=100)