How to calculate interest deduction after cash out refi?

[Jack B.]

Cash out refinanced 4 rentals. Goal is to buy more rentals with the money, but I'm not finding any deals yet. 

How can I calculate the interest amount I'm allowed to deduct on the new loans for the rentals? Are they fully deductible since the money is not being used for personal stuff but set aside for investment into more rentals?

Do I subtract the old interest amount? (That doesn't seem quite right).