But how much has the underlying price been degraded from when...

But how much has the underlying price been degraded from when you buy & until the 10% fall?
BBUS doesn't 100% correlate with the index because of the fees.
ie you buy at $1.00 when the index is at 10,000.
12 months later it falls to 9,000 ie 10% but BBUS won't be at $1.25 (+25%), probably closer to +10-15%.
or
If you buy at $1.00 @ 10,000 & 12 months later the index is still at 10,000 BBUS will probably be around to 85-90c
i.e the index has to fall just for you to break even.

This is why holding short ETFs for the long term is a poor investment.

US markets could keep on rising for another 12-24 months before a correction.