Assume the following:

Value of the dwelling: $1,000,000
Coinsurance: 80%
Required Insurance: $800,000
Insurance limit carried: $500,000
Damage: $800,000

Divide the limit of insurance $500,000 by the Required Insurance $800,000 = 62.50% Recovery Percentage

Multiply the amount of damage $800,000 by the Recovery Percentage 62.50% = $500,000. The insured is paid the policy limit.

If the amount of damage is changed $700,000 the insured only recovers $437,5000.

So you don't need to have a total loss to recover the policy limit you need to have an 80% loss or 20% value remaining.