For 240p systems, dot clock is typically below 10MHz, so first guess would be 32MHz/4 = 8MHz. Looking at some PCB pictures (like this), Cave 68000 boards actually seem to have 16MHz and 28MHz XTALs. The same MAME source file indicates games use 320 or 384 column resolutions. Perhaps the dot clock is 7MHz (28MHz/4) with 320-column games and 8MHz with 384-column games (dots per scanline could be 426/512). The first thing would be to check line counts and Hz from both 320/384 games and then guess further. Alternatively, if you have an oscilloscope, you could measure the clock that drives video DAC if there is a dedicated one on the board.