One hit, hits all

As you know setting a 'percentage expectation' for disruptive events is common in any operating plan, QoS plan, security plan etc. Contingency (forward planning) of something that might/may happen as a percentage factored in advance in order to ensure support being available is standard practice.

The author Henry Basset's 'Red Sky Alliance' records in his blog (http://henrybasset.blogspot.co.uk/