quickest/easiest way to globally adjust the horizontal size?

[adder]

hi, im still in the setting up/learning stages of groovymame and it is going well but i have a question:
my crt tv doesnt have the option of adjusting the horizontal size (personally i do like slight black borders on each side when i play games). what's the quickest/easiest way to globally adjust (pull in slightly) the horizontal size in groovymame for all games? im using magic modelines, and have about 20 modelines in my registry. in mame.ini i have: monitor arcade_15, and crt_range all set to auto
cheers!

[Calamity]

Use the arcade_15 preset as a starting point:

 crt_range0  15625-16200, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 288, 448, 576

Set "monitor custom" in mame.ini. Then play with the remarked values, they represent the right and left borders (yes, this order).

If you set "dotclock_min 8.0" you will get more consistent results.