Is this calculation for an optocoupler's resistor correct?
Your calculation is correct, but your operating point is not well-advised. Note that the absolute maximum for the forward current is 60 mA, so running the LED at 50 mA is not a good idea. 

Instead, look at figures 2-4, and note that the sensitivity of the output (current transfer ratio, or CTR) peaks at about 10 mA. Also note that all the performance numbers are also specified at 10 mA. As a result, you're probably best off if you drive the LED at ~ 10 - 20 mA. Refer to figure 1 of the data sheet to find the nominal forward voltage. And be aware (from figure 1) that this varies with temperature, so you should get some idea of just what environment you'll be operating at.

Once you've picked a current, you can plug the appropriate numbers into your calculation.