In a hypothectical Reg T account with IB, I have $500000. I added up how many shares in SPY would equal this amount. If SPY was at 100, then I could buy around 500 shares which would add up to $500000 total. My Reg T margin requirement would be $125000 I believe. In this situation, am I paying interest to IB on the $12500O to IB? If not, how much more stock would I have to before interest began to accrue ?