Are my mitigation numbers off ?

[CGMidlander]

How do you obtain that number ?

If your shield blocks 30% of damage 15% of the time, it means the average mitigation is 4.5%

At most, Sheltron's "effective" duration is 3s (Less on multiple targets, but never more, since it only blocks one hit). It means that it mitigates 30%, of damage, at most, 10% of the overall time (Duration/Recast) for, at most, 3% average mitigation.
On the 90% remaining time, your basic block rate applies., so 90%*4.5% = 4.05%

So, by combining Sheltron and passive Block, you obtain 3%+4.05% = 7,05%.

The benefit of Shletron is not to mitigate a lot more, but mitigate more reliably.

Using average mitigation is a flawed way of looking at how things work in practice, since the game is largely focused on mitigating (predictable) tank busters. If average mitigation mattered much, then parry wouldn't be regarded as such a crap stat. (Well, it'd still be bad because it scales poorly, but you know what I mean right?)