I'm using web clock config tool to setup the clock and generate clock_config.c and clock_config.h files, and then I export the two files into my project. Sometimes I select a board e.g. "LPCXpresso54618" and sometimes I select a processor e.g. "LPC54605J512" to start with and then I make my desire changes.
I have a 24MHz xtal on my board, so I change the "clock input" to 24MHz and change the "system oscillator freq range selection" to High Frequency.
As far as I can see, everything works fine, but I have noticed that BOARD_XTAL0_CLK_HZ in clock_config.h file is always set to 12MHz. Maybe it is not used any where.
define BOARD_XTAL0_CLK_HZ 12000000U /*!< Board xtal0 frequency in Hz