Hi. I am a new Real Estate Investor 'wannabe" and I'm practicing calculating my numbers for a flip. 

(Using any numbers here)..

If I have a property that's asking $50,000 and the lender requires 20% down, ($10,000) then I'm borrowing $40,000. The lender has a 10% interest rate.

Rehab cost is $50,000 and the lender is covers 100%. I have to pay out of pocket a certain percentage to cover the first rehab draw. The lender will send out an inspector and reimburse me the money I paid on the 1st draw payment.

QUESTION:

When trying to calculate a portion of the "Holding Costs" do I add the purchase price AND rehab costs together ($40,000 + $50,000) = $90,000 and that is what I calculate the 10% interest rate?

Example

$90,000 x 10% = $9,000 in interest for 1 year.   $9,000 divided by 12 months is $750 a month.   $750x7 months= $5,250 total interest. Is the $5,250 for both the purchase loan AND the $50K rehab?

OR do I only calculate the purchase loan amount ($40,000) against the 10% interest rate?

Thank you!!!