Dr Zira, that formula doesn't seem to work for my portfolio.
Buying Power is what you have available to spend on stock. Whenever you buy stock, you pay for the stock plus a commission of $10 ... so you start with $200K of buying power (the beginning of the game, your $100K fantasy money plus $100K available to borrow), and you buy $25K of zit stock, then the buying power left is $174,990. If your stock goes down, say to $23K, and you decide to sell it because you don't want to lose any more, then your portfolio goes up $23K minus the $10 commission ... $22,990. Now you have $174990 plus $22990 = $197,980 ... so your new buying power is $197,980
If it had gone up to $26000 instead, then your new Buying Power is $200980.
Does this help you understand how Buying Power is figured out?
Don't forget that EVERY SINGLE TIME you BUY or SELL, a commission fee is subtracted from your available money.
When you set up an order, either a LIMIT or while the market is closed, the commission is not subtracted from your Buying Power until the order is filled. If you cancel the order before it is filled, or if it expires, then no commission.
GOOD LUCK to all!