Basically BTC-e is more or less determining the conversion rate FTC/BTC, as they have the biggest volume of FTC related deals at the moment.
The price is determined more or less as follows:
BTC-e gets buy orders of different value and size, e.g. 1000 FTC at 0.0034 BTC; 100 FTC at 0.0032 BTC and so on.
On the other hand people want to sell FTC so BTC-e gets sell orders, e.g. 200 FTC at 0.038 BTC, 1000 FTC at 0.0040 BTC…
Sellers always want more money than buyers want to pay
If a buy order and a sell order match the deal is performed and that is the exchange rate for the moment.
If there is no deal, because there is a gap between buy and sell orders, they apply some math and define the exchange rate.
It’s basically exactly the same as for FIAT exchanges.
There is no fixed formula for this, so the only way to get a rate is to use the API of the different exchanges, retrieve the exchange rates and choose the exchange best matching your needs for the next deal.