If your free margin drops to zero, your broker will send you a margin call in order to protect the used margin on your account. Always monitor your free margin to prevent margin calls from happening, and calculate the potential losses of your trades (depending on their stop-loss levels) to determine their impact on your free margin. With some experience, you’ll find it significantly easier to follow your margin ratio and understand the meaning of margin in Forex trading.
I post this to let you know, as the title mentions it, that I made a trading diary, with google documents tool. This a generic spreadsheet which allows any trader to manage his trading (his risk, his pnl, his opened position, the orders...) with a trding diary. Every trader,should have one, and I mad mine with google docs. At least you must have an account to acces this spreadsheet.