Are my mitigation numbers off ?

[Reynhart]

On average your Sheltron increases mitigation by 26% so it's still an excellent skill.

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.