Hi-

Re. colorbar and overlay:
Let's say the max colorbar max value is set to "A" (and A>0): this could be the max value in the dset, the 95% percentile in the dset, or any number of your choosing (you unselect the 'default' one and type it yourself). Then, the possible values of the *bottom* of the colorbar are either "-A" ("Pos?" box is unchecked) or 0 ("Pos?" box is checked).

To have a lower bound that is not -A or 0, you can, indeed, use 3dcalc to offset things, and have an effectively arbitrary lower bound.

For the *underlay* (possibly a sidenote):
You can set the gray/white (or other) colorbar to go from [B, C], where the lowerbound B is *not* constrained to be 0 or -C. To set that, right click on the vertical grayscale cbar in the image panel, select the upper "Choose Display Range", and put in 2 numbers; note, it will only change it for that image panel.

--pt