Depends on how strong you want it to bubble.
I'd suggest keeping it under 3 volumes personally, but a lot of people go from 2-2.5 volumes.
On the calculator you generally pick the co2 volumes (sometimes they list various styles of beer, but have the volumes in the description as well).
Enter the size of your batch (usually in gallons).
Then some ask for the temperature of your brew to determine how much co2 is currently dissolved.
You select what kind of sugar you are using.
Then it tells yu how much you will need for the whole batch. This is great if you're using a bottling bucket because you can disolve the sugar into a small amount of water, toss into the bucket, then syphon the cider on top of it and it'll mix itself as it fills up. This also ensures an even distribution of priming sugar throughout the batch.