I thought I would be fancy for my current project and set custom chapter breaks in my DVDs. I counted out the frames using Avidemux and Vitualdub, unchecked "Use Source Chapter List" in my preferences, set the Chapter Interval to 999, and then opened my View/Edit Title Settings, went to Chapters, deleted any previous auto generated chapter breaks, and added new breaks with my frame counts. Unfortunately, the frame values were resulting in bad (too short) times. In one example a chapter break at frame 31893 corresponded to a time stamp of 0:22:10.205 in Virtualdub, but in the custom chapters window of AVStoDVD, frame 31893 was showing a time stamp of 00:17:44.164. Obviously, this mistiming is thoroughly throwing off my chapter breaks when I play the DVD. Not surprisingly, the automatically generated chapter breaks have similarly messed up timing, for example the auto generated chapter break for frame 18000 shows a time stamp of 10:00.601 where in my source videos, frame 18000 corresponds to a time stamp of 12:30.751. And if you are at this point wondering, my source material clocks in at 23.976 fps.
Aside from whatever is the root cause of this issue, it seems entering custom chapter breaks via time stamp (something, for example DVD Styler can do) would be a straightforward way of sidestepping the issue. It would also allow an easy way to add "close enough" chapter break values without having to open up a specialized application in order to count frames.
Thanks for your attention.
PS -- After working on this some more this morning, the problem is indeed caused by my source material being 24fps where AVStoDVD is assuming it is 30fps. I can work around the problem to a certain extent by taking my frame count and multiplying it by 1.25. This stops working if I try to put a chapter break near the end of a video as my multiplication can produce a number of frames larger than contained in the file, and the too large value gets flagged as an invalid figure. So, there you are.