First, it is a very bad idea to quote numeric results, "numeric text" like
"100%" as Luke did. It is often the root cause of many problems later on.
The better simple formulation is:
=if(A9=0, 1, C9/A9 - 1)
formatted as Percentage.
But since A9 and C9 might be negative, I suspect you would be happiest with:
=if(A9=0, SIGN(C9), (C9 - A9) / abs(A9))
This has the following results for various values in A9 and C9:
-50 -50 0.00%
-50 -25 50.00%
-50 0 100.00%
0 0 0.00%
0 50 100.00%
50 0 -100.00%
25 50 100.00%
50 50 0.00%
-50 25 150.00%
25 -50 -300.00%
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