I have a sperical reflector bowl to use in my pj. I've read in the reflector thread that the arc should be slightly in front of the focal point of the sphere. Does that mean A or B in the picture below? I'm planning on placing the arc about 10mm from the center of sphere.

Also, is lamp placement based on the centerline of the arc tube itself?

As I understand it the sweet spot is your picture A. All placements refer to the arc itself - that area between the anode and cathode of the bulb. Like where the tungsten filament would be in an incandescent bulb - eh?

That said, most people find tweaking relative placement of components generally makes considerable improvements in the projected image so be sure to allow for some adjustment.

I have one of the original Norpro reflectors and have tried it in just about every conceivable placement. By far the best spot is closer to your diagram "A."

See this too.

I saw this post, which makes sense. I just was not sure where to put the bulb in relation to the reflector. I will have everything slightly movable to adjust.