Investors and lenders often use the 70% rule to decide the highest acceptable price to pay for a property while still making a profit. This helps you and the lender understand how the after-repair value applies to the overall property value and your potential loan amount. To use the 70% rule, first estimate the after-repair value of the property based on local sales figures of similar properties in the area. Multiply that figure by 0.7, or 70%. Now subtract the cost of estimated repairs, somewhere between $15-60 per square foot. With this new value, you have the maximum buying price you can pay for a property to still make a profit.