You don't bother calculating it.
You plump for a figure that sounds OK, for you, and also carries the level of impact on the price of the policy that you can handle... knowing that we are talking lightning strike odds here.
Thanks Hippogriff! That seems a reasonable approach, especially after I asked the insurer to quote for a higher rebuild amount and their quote was basically twice as high for twice the rebuild cost. That premium seems exceptionally poor value to me, given that the bulk of their payouts must surely take the form of smaller sums for lesser damage, rather than a total rebuild. You're right, it is very very unlikely that a complete rebuild would be needed - only a major house fire would do that.