"Brett Romero" wrote...
....
If both numbers are negative and the change is increasing in a negative
direction (as above), how can I flag the resulting cell as red? If the A1
had been 5000, the resulting cell would remain black.

You can't without referring to the denominator cell itself. If that's OK,
then base conditional formatting (Format Conditional Formatting) on the
sign of the denominator.

My point was that ratios are inherrently ambiguous because +/+ is different
than -/-, as is +/- vs -/+. There's no way to make sense out of ratios
(percentages) without knowing the sign of the starting value.