Steve, 

Although it sometimes works out, it is oversimplifying to assume that if a property is insured for 50% of its' value, losses will be paid at 50%.  That is the idea behind coinsurance, but you have to use the coinsurance formula to get the correct answer every time.  

Also, when using the coinsurance formula, you do not subtract the deductible.  The deductible is assumed in the formula.  

If a property is not insured to at least 80% of it's value, you use the formula on page 29 in the state manual to determine the amount payable in a loss. 

Jan