Honestly, I don't have that statistic and I wouldn't invent one. But I don't need it — the directional logic is enough. Fast digestive tract, enthusiastic digger, swelling substrate. The risk profile is unfavourable regardless of the exact percentage.
Final recommendation: Get a bag of
Komodo Calci Sand or, better for this purpose, a bag of kiln-dried children's play sand — the
Gowi Toys Kinetic Sand or any dust-free, additive-free play sand from a garden centre works perfectly. Fill a low-sided storage box, something like a
Really Useful Box 9 litre, and you've got a safe, satisfying, permanent dig station.
It won't swell, it's inert if swallowed in small amounts, it mimics natural digging substrate, and it'll last indefinitely with an occasional top-up. For a chocolate ferret who loves to dig, this is the right setup — safe, cheap, and genuinely enriching.