This doesn't look right to me, compare the following:

```
In [55]: actual = -1j

In [56]: expected = 1j

In [57]: abs(1 - abs(actual) / abs(expected))
Out[57]: 0.0

In [58]: abs(actual - expected) / abs(expected)
Out[58]: 2.0
```

I think it should just use the latter.