Actually, I overstated the value of Hold the Line a bit in that part due to the nature of the first test results.
As for calculating it, presuming it is two-roll as it appears, Hold the Line will convert an additional 10% of your Blocks multiplied by the uptime into Critical Blocks.
Against a single target, the uptime is around 55%. Against multiples, it is around 100%.
If your Block Rate is 45% against a single target and the uptime is 55% of Blocks, you gain 2.5% Critical Block, presuming the combat table is kind to you.
Each percent of Critical Block is only worth 30% damage reduction per point, meaning in that scenario Hold the Line is only giving you 0.74% damage reduction, or 0.37% per point.
The 0.75% approximation per point is presuming a best-case scenario of uptime.
The problem with Hold the Line is that with the innate volatility of a two-roll system, the rate at which you Block will have a massive impact on how many Critical Blocks you get.
For instance, if you notice my lower Block rate against the pack of three, you'll see that the gains from Hold the Line are next to nothing, whereas with my higher Block rate against a single target the gains were not huge, but at least noticable.
(Hold the Line also will wildly fluctuate in value against a single target depending on if you align your Shield Block cooldowns with Hold the Line being active or not. This is less of an issue against multiples given the high uptime of Hold the Line.)