Did some experimenting with Block strength using 3 different shields (Onion 92, Holy Shield Zenith 181, and Maelstrom 6) utilizing both a DPS STR (423 STR) setup and a typical Tank setup (358 STR), and no gear aside from an Allagan Sword (233) STR.

In a STR setup the HSZ reduced 28%, Onion 21%, and MS 13%, with Parry negating 25%.

In a Tank Setup the HSZ reduced 27%, Onion 20%, and MS 12%, with Parry negating 23%.

In no gear setup the HSZ reduced 25%, Onion 18%, and MS 10%, with Parry negating 20%.

Firstly STR affecting damage reduced is horrible, at almost 190 STR difference between extremes we saw a partly 3% difference with shields and 5 with Parry. Second since something like the Maelstrom shield with nearly no block strength is still blocking 10% there's probably a natural base reduction (probably between 5-6%) for all shields.

After doing the math I needed about 68 STR (give or take a point or two due to gear constraints) to reduce 1% damage on a shield, being I'm about 224 STR naked with the MS shield divide by the 68 it comes out to about 3.29%, since it's rounded down it will be 3 so it appears my theorycrafting about a absolute minimum negated is true for 7%.

Now onto the Block Strength of the shields, we got a fix on just how much STR affects the damage reduced we can now get a better estimate on how much block strength calculates point-per-point.

For the sake of shortening this discussion 423 STR /68 = 6.2 mitigated from STR, round down 6 + 7 min block value = 13% base mitigation.

28% HSZ - 13 Base/STR mitigation = 15% negated from Block Strength on the HSZ.
21% Onion - 13 Base/STR Mit = 8% negated from Onions Block Strength.
13% Maelstorm - 13 Base/STR Mit = 0% negated from the MS Block Strength.

Now here's where things get a little cloudy, since I don't have slight BS increments to work with I have to rely on some rough calculations.

181 HSZ / 15% = 12
92 Onion / 8% = 11.5

To add to the formulas I'll throw in the regular Holy Shield's Block Strength and Mitigation to help us pinpoint a more accurate number.

167 HS / 14% = 11.9

So it seems our ratio is between 11.5-12 Block Strength per percent mitigated, but since we have the residual output we should be able to reverse engineer a fairly accurate answer.

HS 14% * 11.5 = 161
14% * 11.9 = 166
14% * 12 = 168 (over)

HSZ 15 * 11.5 = 172
15 * 11.9 = 178.5
15 * 12 = 180

Onion 8 * 11.5 = 92
8 * 11.9 = 95.2 (over)
8 * 12 = 96 (over)

Assuming there isn't a scaling value to Block Strength (IE: You need more BS for every % mitigated), 11.5 points per % mitigated appears to be the correct unit of measurement, but I would like to get some more data from more mid-level shields just to be sure.