system clock frequency - any reference?

250 MHz didn't seem to work on raspberry pi 4 with baud rate set at 115200. However, 500MHz did. Do you have any reference to how to set those clock frequency? It seems like the number we are setting here is different from the frequency found when we run vcgencmd as found here https://forums.raspberrypi.com/viewtopic.php?t=211469 (this of course was done after redownloading raspberry pi image and using cli inside raspberry pi) So what is the clock frequency that we SHOULD use and how do we find them?