Yeah that should work. I'm pretty sure you can piggy back any of the fuses, as long as it comes on when you need it to.
Thinking about it, I did something similar for my dash cam. Here's a pic of my driver's side fusebox, with the trim removed:
Piggy back fuse is the one with the yellow fuse (circle #1)
This goes to the positive (red) wire of the converter, which is the black box to the right of the black conduit (circle #2)
Black wire goes to ground as you mentioned. In this particular case, I used a nut I found just above the fuse box. I undid it and used a crimp ring terminal to attach the ground wire. You can just about see the nut (circle #3)
One thing to note when using the piggy back fuse taps, is that orientation does matter. It shouldn't be detrimental, but if you insert it the wrong way, the original fuse will carry the current for the new circuit as well. So depending on how much current the new circuit draws, it could lead to the original fuse blowing.
Here's a simple pic I found online when I was doing it. The live side should be the left side.
Easy way of checking it would be to stick in the piggy back fuse tap without the original fuse. If you've orientated it correctly, your new circuit (tablet) should receive power, whereas if you've got it the wrong way around it won't. Once you know which way to orientate it, don't forget to stick the original fuse back in
Although I would double check on your own car, from my pic above, it looks like the top row of fuses has the live (12v) on the bottom side.
Hopefully that makes sense?