Vuescan focus batch-scanning slides on Nikon LS-8000

I would like to scan batches of 5 mounted 35mm slides in my Nikon LS-8000, using Vuescan. I am confused about how best (and easiest) to achieve good focus.

The simplest thing is to set the focus point X/Y midway between an edge and the center of the transparency, to "split the difference" on a "cupped" slide. Then (in the "Input" tab) set "Auto focus" to "Scan", and "Batch scan" to "All". As I understand it, Vuescan will have the scanner focus each slide at the specified X/Y focus point, before scanning it. This would be a decent solution, except for times when, for a particular slide, the focus point lies on a part of the image with no edges or texture for the focus algorithm to use.

The way to get the best result seems to be to step through the five slides individually. On each one, first do a preview, and then manually set (by moving the focus "cross hair" icon) the focus point to a spot that is not too near the center of edge of the slide, typical of the most interesting part of the image, and with some texture upon which to focus (such as an edge between very different brightnesses). Then scan the image with "Auto focus" set to "Scan" so that focus is done at the individually defined focus point, just before the scan. This seems to give good results, but does not allow batch scanning (the user must manually interact with Vuescan between each slide) – and this scanner is SLOW.

I thought perhaps it would work to batch preview the slides, then manually set the focus point for each one, and then batch scan. But it appears to me that it doesn’t actually use the 5 distinct focus points for the 5 slides, but rather just uses the first one. So this isn’t the solution.

I wonder if others have dealt with this problem, and if so, how.