ok, thanks, I did not understand this formula, and tbh, I also don't understand yet what your code example actually does.
But IIUC, on the other serial ports >=Serial1 every single baud rate of the above mentioned >=230400 is expected to work, just with Serial.begin(x) nothing of them (and perhaps also nothing very close to any of them?).
If so, then that's a pity, because I use Serial() via the USB port to the 2nd device.