Setting prescaler for 8MHz external oscillator in Atmega2560
Datasheet recommends baud rate error below 2% under standard 16x oversampling mode. So with a 8 MHz clock, 57600 is not achievable, error is more than 3.5%, which is too close to the max total error of 4.5%.
Now, using U2X mode for only 8x oversampling, 8MHz clock can achieve 57600 with error of only 2.1%. Unfortunately it still exceeds the recommended error of 1.5% of U2X, even thouh it is less than max total error of 3.9%.
So, 57600 is not achievable with 8 MHz clock, it is not recommended by manufacturer.
However 38400 and many other baud rates are achievable with much below 1% error.
If you are able to change the clock to something more suitable like 7.372800 MHz, it is possible to achieve bit rate of 230400 with 0% error.