Hi BeeryGaz,
That's very easy to achieve. Just grab the generic_15 preset from here:
http://forum.arcadecontrols.com/index.php/topic,116023.0.htmlcrt_range0 15625-15750, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0,
192, 288, 448, 576
As you see, it is defined to cover the range from 192 to 288 progressive lines. Just modify it as you like, e.g 192 to 256 lines:
crt_range0 15625-15750, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0,
192, 256, 448, 576
This line goes into mame.ini, obviously. Remind to edit the "monitor" option in mame.ini, to
"monitor custom", so that the custom crt_range line has effect.