Hi Andrew,

If your desired output for this input:
-1%
-1%
1%
-2%
-2%
6%
-1%
-1%
7%
would be:
0%
0%
0%
0%
0%
1%
0%
0%
5%
then enter into cells B1:C2 (normal, NOT array-entered):
=MIN(0,A1) =MAX(0,A1)
=MIN(0,A2+B1) =MAX(0,A2+B1)
and copy B2:C2 down as far as necessary.

HTH,
Bernd