I'll be using my own number of rejuv ticks for the examples here, which I get 9 ticks with only 1 Persistence relic and 7000 something haste.
So, at a 2% proc rate, there would be a 98%^9 (0.98^9) chance for it to NOT proc, which is 0.8337, or a 16.63% chance for an individual rejuv TO proc with the 4p over the course of it's entire duration. Meaning, with no waste (combination of Germination and it being a smart heal would help towards that), you would get a 16.63% healing output increase from your rejuv.
Now, at 1.5% proc rate, you get approx 3/4 of the previous. 0.985^9 = 0.8728, or a 12.72% chance proc rate, eg a 12.72% increase on rejuv healing.
All-in-all, it's worth (for me, at 30% rejuv healing), about a 3.8% healing increase. I might mess around with Germination, but obviously, on fights where tranq is super important, you should likely use that instead.
Worth noting, the legendary shoulders DO work on the 4p. BUT, with it being a smart heal, it's not really worth it to use, imo.