ahofle: I've not played DoDonpachi before. It reminds me of ESP Rae. That mode you created does work, almost without need for adjustment, though it ends up running at 71.1hz (which doesn't seem to affect game play). I don't know how you get '@60hz', though, because when I calculate it, it doesn't happen. Actually the refresh will be around 73-75 if wanting 25khz.
I created my own modeline which comes closer to 60hz (as in Strikers1945 there's some stutter using yours), but the scanrate is also lower at 20.9khz, which the Betson is probably not capable of. Still, for anyone using a Billabs, or perhaps any other monitor that might be similarly capable, the modeline is:
modeline '432x324 20,9khz 61,29hz' 11,20 432 464 504 536 324 330 334 341 -hsync -vsync
By the way, instead of using a custom25khz.txt, I put them in the usermodes.txt - with which I guess modes of any scanrate can be added.