=(B1-A1)/ABS(A1)

=if( A2=0, "", (B2-A2) / abs(A2) )

Since you are dealing with mixed sign, it seems prudent to do
__something__ with A2=0.

The problem, Patrick, is that the percent change is not particularly
meaningful, in my opinion, when the underlying value is negative (or zero, for
that matter).  The absolute value is meaningful, and the change from loss to
profit is meaningful, but percent change is not.

For example, given the same positive Current value, and using, let us say,
Bob's formula, you will note that the profit becomes greater as the Prev
approaches zero.  This implies that the profit has gone up by an increasing
percent, when the amount of profit has decreased, and the baseline has
increased:

Prev   Curr    Bob's
-5    10    300%
-1    10    1100%

In financial reporting, in these circumstances, a percent change is not
reported by the few sources I've used.

Reuters only reports percent change in earnings if both periods have positive
earnings.  Otherwise it reports (NM) (not meaningful).

The fact of P(rofit) or L(oss) may be reported.

From WSJ.com HELP:Digest of Earnings
http://online.wsj.com/public/resources/documents/doe-help.htm

"Net Income percent change is the change from the same period from a year ago.
Percent change is not provided if either the latest period or the year-ago
period contains a net loss. On the digest page, if a company posts a profit in
the latest period against a loss in the year-ago period, the percent change is
represented as a "P". Similarly, if a company posts a loss in the latest period
against a profit in the year-ago period, the percent change is represented as a
"L"."

For what it's worth, I've been told by a math PhD that percent change with
values of opposite signs is not defined, but I've never been able to find that
documented anyplace.
--ron