You would not want to downclock it at all. Run it at 3600 or higher so the Infiniti Fabric is at 1800Mhz or higher, some people like me have found that you can get a stable 1:1:1 FCLK:MEMCLK:IF at 1900Mhz and others maybe only 1866. 1800Mhz though is where AMD says the shine for the IF. But that is just my opinion.
MSI MEG X570 Ace
AMD Ryzen 9 3900x
Corsair H115i Platinum, Corsair Crystal 570x RGB Case
32GB Corsair Vengence RGB Pro 3600 @3800
MSI Geforce RTX 2080 Super Ventus SX OC w/ Acer Predator X34P