Today I encountered a client with a blended loan. The LOC was used for a deductible use and since 2014 the client keeps drawing additional private money from time to time. So it is required to apportion the loan interest and monthly fees. And the deductible % is continually changing. Here is the excel calc of this loan. Such a calculation would quickly satisfy the ATO that a reasonable basis is being used to determine tax deductions and that it is being changed. Note how repayments must also be apportioned across both purposes and the % is updated too.