The headroom is now in fact an optional keyword setting in the function:
```
def _get_update_rootps_whitening(self, shape, dx, headroom=1.05):
```
The reason for 1.05 is really just gut instinct.  1.05 corresponds to ~2.5% in SNR, which seems pretty much negligible to me in real terms. And yet 5% of the added whitening noise is a pretty healthy margin to play with in terms of numerical precision in all cases I can envisage.  I wouldn't be so sure about `1.0005`, and it doesn't really buy very much.