If you're taking Proviron with a test-only cycle, then it's a very synergistic combo because Proviron reduces aromatization and also increases free test since it binds strongly with SHBG.
If you're taking Proviron with a test & deca cycle, deca will bond more strongly to anabolic receptor sites, but Provirion is still 3-4 times more androgenic than test (being a DHT-derivative) and it still binds strongly with SHBG, thereby increasing free test.
If you're taking Proviron with test & tren, well, hardly anything is as androgenic as tren...so Proviron can't compete there. Nor does tren aromatize, so you don't need Provirion for that. Test, however, does aromatize...so Proviron can help there and still bind to SHBG to increase free test. The only concern in this case is if you have hairline or prostate issues. This is a heavily androgenic cycle, so it could cause you problems.
Oh yah, 50mg is a nice, effective dosage for Proviron.