I have a basic question about initial margin requirement and hope someone can help me. I have a regular margin account with IB and have $20000 in the account , and suppose spy was trade at 100 yesterday and I bought 200 share of spy at this price. If tomorrow I want to buy another 50 shares of spy, do I violate the initial margin requirement since the cash in my account is zero? If so, can I work around by first selling the 200 shares of spy that I already have and then long 250 shares of spy? Many thanks in advance.