Your memory card is one space, the onboard area another. With the space onboard so low, that Calculating may never come back.
You didn't reveal exactly which Galaxy Tab so I can't guess if you can add another memory card. My model SM-T3100MKAXAC for example has 16GB onboard which gives me a bunch of space.
Here's a generic article about clearing Andorid cache space.
Android 4.1.2 looks pretty old. What is this model?
I was trying to download an app on my Samsung Galaxy Tab, but it said I have insufficient storage space. So I uninstalled a few apps but the available space on my device stayed the same and I still can't install the app. When I view my device storage, it tells me my total space is 8.00GB and my available space is 131MB. The other details (space taken by applications, pictures/videos, audio, etc.) just say "Calculating..." and have been for the last 15 minutes. I can't install anything and I don't know what else to do.
I'm running Android version 4.1.2