Mastery (100% pickup rate, T18 HFC 4pc using PoM): 0.45
Mastery (typical pickup rate, T18 HFC 4pc using PoM): 0.25
Divided by 1.25 since Extend Life healing isn't increased by Mastery (but increases with everything else, so it's the only stat that changes) and applied a new 10% penalty for uncontrollability/unreliability (was 0%, but people aren't bothering to pick up orbs like they were in SoO because they heal like 0.5% of max HP each instead of like 10%, so I should have applied a penalty ages ago for that). Not complicated.
You may be seeing lower numbers than the stat weight indicates in your logs, but keep in mind about 10% of your Mastery's worth is hidden in your personal Leech and Etheralus absorb healing. Or maybe the pickup rate has just gone way down. I'd need to do more research before I conclude that, though. Hm, let's see:
According to this theorycraft, 19% Mastery should give you +3.8% healing done (3.67% of total healing). Then in logs if you have about 10% of healing from your own Leech and Etheralus absorbs, it should show as about 3.3% of total healing. Let me know if that's not lining up with logs.
(Edit: unless your pickup rate is well over 100%, which is the multiple-pickup bug they still haven't fixed since Throne of Thunder. Example log with 139 spawns and 172 heals)
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5.28... Multistrike is 5.28 times as good as Mastery.