First off, hello and Happy New Year! 
I'm new to CHDK but I have been playing primarily with the motion detection scripts.
For the last couple of days I have been trying to photograph fireworks on a G15, F5, 6sec exposure. My issue is the focus - if I set MF and then rail the focus right to the end the focus is very out 
its nowhere near infinity (I would assume its a long way beyond for a reason below) I haven't been using CHDK for this.
If I set the camera to AF and take the same shot, and its nice and sharp. The camera is screwed to the balcony railing so it isn't moving.
Is there an 'easy' way to figure out where infinity really is in terms of focus so that I can have a script which simply preps the camera for fireworks? I wrote a simple script which sets the focus at 65535 and then shoots, reduces to 65534 shoots etc....
But that's going to take ages so while the cameras happy snapping away is there and easy way to get the camera properly focussed at infinity? With or without CHDK?
I really don't understand why you are having problems getting the G15 to focus at infinity using the built-in MF capability. But what happens if you set the focus at 10 of 15 ft and take some night shots ? (Those are the MF distance steps before infinity on my G10 - the G15 might be different).
The reason I ask is that the little lens systems in P&S cameras have amazing depth of field. Typically, if you focus a few feet away from yourself, everything from there to infinity will also be in focus.
So I'd suggest taking some more test shots at night in Canon MF mode as you gradually adjust the setting wheel back from infinity and see what the images look like.
If you really want to use a script, the set_focus setting is in units of millimeters ( 25.4 mm = 1") so changing the setting in steps of 1 mm is not really useful. I'd suggest simply using a few discrete settings : 1000 5000 10000 20000 30000 40000 50000 and see what happens.
More info on CHDK manual focus is here :
http://chdk.wikia.com/wiki/CHDK_Manual_Focus_and_Subject_Distance_Overridesand there is an active thread here right now :
http://chdk.setepontos.com/index.php?topic=11078.0Both are still a "work in process"
Update : I took a look at the EXIF data embedded in your images. Other than the timestamp and filename, the only difference between the two images is as follows :
IMG_2921.jpg
Focus Mode : 0x0008
Focus Distance Upper : 1617 mm
IMG_2922.jpg
Focus Mode : Continuous
Focus Distance Upper : 6553 mm
which I believe tells us that the first shot was in MF mode and the second in AF mode. AFAIK, Canon EXIF do not contain the actual focus distance - what they give you is an "in focus" range via Focus Distance Upper (FDU) and Focus Distance Lower (FDL). The MF shot had its FDU set at 1.6 meters (~ 5 ft ) while the AF shot was at 6.6 meters ( ~21 ft)! So it looks like your MF setting is actually
short by a quite a bit rather than too far out as you originally guessed.
Note that on some cameras, the MF setting works in a dual range mode - you have to keep rotating the dial to get it to switch to the longer distance settings ...
