Re: boundary conditions for relaxation method

["John S. Denker" <jsd@MONMOUTH.COM>]

Michael Bowen asked about:
...
> I noticed a cool thing, which is that as the spreadsheet recalculates to
> accommodate changes in the central potential, the color-coding of the
> vacuum cells changes in concert with changes in the numbers.

A hint can be found in the second-to-last paragraph of the
instructions:

  http://www.monmouth.com/~jsd/physics/laplace.html

     I use "Conditional Formatting" to highlight
     the contours of potential and field-strength.
     The contour interval is controlled by a cell
     near the lower-right corner of the potential
     grid.  A value about one sixth of the maximum
     potential-difference seems to look good.

If I were you, I'd click on a cell and then use
the Format (custom formatting) menu item and do
some reverse engineering.