The NFA Required Margin/Security Deposit on Majors is 2% of the notionally traded amount.

A Major currency pair as described by the NFA must be comprised of 2 currencies both of which must be from the following list: AUD, CAD, CHF, DKK, EUR, GBP, JPY, NOK, NZD, SEK, and USD.
Examples: USD/NOK, EUR/SEK.

All other currency pairs have a required margin of 5% of the notionally traded amount.
Examples: USD/MXN, EUR/HUF

Notional Amount needs to be in USD terms.
If USD is base currency
Notional Amount = amount traded

If USD is Quote Cuurency
Notional Amount = Amount Traded x Rate

Updated example:
The Trader buys 100,000 GBP/USD at 1.9505 (using 2% as the Required Margin).
(Traded Amount X Rate) = Notional Amount, Notional Amount X Margin Percent = Margin Used, or
(100,000 X 1.9505) = 195,050 X 2% = $3,901.00

The Trader calculates a potential profit and places a limit order of 1.9620 (Potential profit of US$1,150.00)
The Trader calculates a potential loss and places a stop at 1.9475 (Potential loss of US$300)
Within the next hour the GBP/USD rallies to 1.9592, stalls and begins to reverse.
The Trader decides the limit will not be reached, so he closes his position by selling the GBP/USD at 1.9558 and locks in a profit of US$530.00

(Notional Amount X Margin %) = Margin Used, (Profit / Margin Used) = Return on Collateral.
$530 / $3,901.00 = 13.59%