I don't use an exchange website. I get my conversion rates directly from the Cuban Banco Metroplitano website. These rates are the exact rate that you will get in Cuba for your exchange. The website is updated daily with that day's rate. They do fluctuate slightly depending on a "basket" of world currencies that the CUC is valued against.
You can also get rates at the Banco Central de Cuba website here.
Note that at both these websites you will see posted both a Buy and Sell rate for currency conversions. Most of the internet conversion websites use what is called the mid-market rate, which is the midpoint between these two rates, which unfortunately is not what you'll actually pay at a bank. That's why I use the Cuban bank websites.