Thanks Mark, that does help! It also exposed some strange behavior with both Nikon Scan and VueScan.
With Nikon Scan 4, I can manually tweak the offset for each frame (I tried a strip of 4 frames) individually to get a centered preview. I found that the delta from one frame to the next was between 16 and 17 units (not sure what they are?). The offset of the first frame is whatever it needs to be based on how the film strip was loaded. For example, I can have offset of 6, 22, 39 and 55. So far so good.
If I then happen to click on the little thumbnails icon in image frame selection window, things get messed up again. That's because it has taken the "6" I entered for the first image and used that location as "0". All other offsets are then applies on top of what where it thinks each frame begins. By the time we get to frame 4, it's now way off. So now I have to manually reset the offset of each frame to 0 and everything works out ok. You would think that all one should have to do is give the initial offset for the first frame and that would be it. Weird.
VueScan was better, sort of. I need to use it more to get the feel for it. Right now, it seems to always scan more real estate than necessary (wastes time). But at least it detects the image data and gives me a reasonable first frame. But it gets confused with subsequent frames and rotates the highlighted image selection rectangle 90 degrees as if it lost track of the image. And I can't just enter a desired frame number and have it get there. It's a similar problem to Nikon Scan. The workaround is to manually lasso the area wanted but that seems crude. It shouldn't be this hard!