I wrote:
But if you can have mixed positive and negative results, I
would also cover the case where "start" is zero, e.g:
=IF(B1 < A1, -1, 1) * IF(A1 < 0, ABS((B1-A1)/A1), 100%)
Of course, the choice of 100% is arbitrary.

The following is better, in case both A1 and B1 are zero:

=SIGN(B1-A1) * IF(A1 < 0, ABS((B1-A1)/A1), 100%)