The margin requirements (whether you have Reg T or Portfolio Margin) only determine how much you are allowed to borrow. Regardless of the type of margin account you have, you pay interest on the actual amount you borrow. So if you buy $200,000 of equities and your account has a value of $100,000, you're borrowing $100,000. But if you buy $2,000,000 of equities and your account has a value of $100,000 you are borrowing $1,900,000.