I don’t actually know if what you want to do is possible, and I say that as someone who spent a full week fighting the decal system for a convincing water effect and ultimately cutting the transparency.
The core problem as far as I understand it, is that decals can’t support what is essentially a dual-normal effect, as they are blending with the GBuffer instead of actually suplemenbting it. What I’d try if I were you (and what I would have done if I had the time) is replace the decal with a transparent quad that has your material setup, because that can at least support the visual effect (although almost certainly at greater expense).
Substrate addresses this to a certain extent, as their new model of transparency decouples transmittance (how much light penetrates/scatters through a material) and coverage (whether or not a material is affecting a given area).