Oh, boy, if it's Kodachrome then "Welcome to my own personal
nightmare!" ;o) I've been wrestling with Kodachromes, first, on an
LS-30 and, now, on an LS-50 for over two years!
While the LS-50 does improve things a lot it still comes up short and
the notorious "blue Kodachrome cast" still needs to be corrected. Like
many others, I've also been looking for a sure-fire, objective way to
set the gray point because, as you discovered, relying on one's eyes
isn't always easy.
In the absence of obvious grays like sidewalks, grays usually hide in
shadows so simply clicking on most shadows will be quite effective.
One "trick" I use (in Photoshop) is to have a go at several images in
a roll (implying, they were taken under similar lighting conditions)
and then "average it all out". I save this as a curve setting and then
apply to all. But like all other tricks it's still a hit and miss...
Another is, that Kodachrome needs red boosted (this works better than
decreasing blue) so try playing with the red curve.
Scanning "raw" is an approach where you only use the hardware settings
of the scanning program and disable all the software settings.
Specifically, it means using the maximum bit depth i.e. 16-bits (or 14
to be exact) and disabling curves and other adjustments. ICE is
in-between because it's part hardware and part software, but since it
can't be done later due to the hardware element, I use ICE as well.
Warning: ICE doesn't work with most Kodachromes! (BTW, VueScan doesn't
have ICE but an inferior "IR cleaning" method which even fans admit
falls way short of ICE.)
The resulting image of scanning raw is your "digital negative". After
that you can do all the editing outside of your scanner without having
to re-scan. If you're not happy with the result, you just start again
with the "digital negative". That's not only faster than re-scanning,
but re-scanning doesn't really bring anything new to the image. And
your scanner will last longer!
Finally, check the archives because there have been quite a few
messages in this group on scanning raw.
Don.