Calculate No Of Lots

Does any body know if there is an example of calculating the no. of lots to trade on a cross currency fx. Assuming account is in USD and I am trading GBP-YEN and I want for example to have a stop loss of 50 pips which will be a 2% loss on the fund if it gets stopped out?

I can do the calculation but but what is the best way to get the fx to base rates

Thanks

Paul