Divide the dividend by the number of contracts and then multiply by the amount of your contracts.
Can't I find that in my history, like any other transaction, including interest received?
Its under Account settings, History, then click on BTC and you will see time stamped dividends for mining. I haven't seen an interest payment for swaps yet so not sure where that is, but I assume somewhere under that history section.
Here is a link.
https://www.bitfinex.com/account/ledger/btcOh, I went back to some older swaps for btc, those payments are also in that section.