No i don't use HLFIX for anything i do. If i can't create a valid brush, then i don't.
I have not tried making a dome with the arch tool. But i think i might know how to do so... in-fact. I'm gonna try right now.
Redka, the way i do it is..thusly: Since a 12 sided cylinder has 3 wedges per 90 degrees, as captain terror mentioned, one of those sides HAS to be a 45 degree angle.
We all know that Hammer doesn't like anything that's not a 45 deg, or 90 deg angle. So i only make the 45 deg angle brushes at different angles at either end. If that confuses you... here's a pic explaining what i mean.
You can see from this pic that the top, bottom, left, and right sides, when flipped, will all be facing in a good direction. And as for the 45 deg angle, as long as you make the sheer in each direction symmetrical, there will be no errors in your map.
So. 8, 12, 24 sides.. are all good to use for curves and domes and spheres and such. Anything that's going to be hollow, anyway.