I'm using GroovyMAME so I just adjusted the monitor specs to set the width.[IMG]https://uploads.tapatalk-cdn.com/20180607
That's a good idea, could you share your monitor specs (modeline I guess to use in VMM)?
BTW, I found a kind of workaround, adjusted the voltage pot a bit which decreased the horizontal width to match the screen. Doing this I also lost some vertical height, but not much. However, I think your method is the way to go.