Import/Grid/Tile settings question

I've been playing with tilemaps, and I'm really liking what I see, but I have some questions about what my settings should be. This may well expose my newbie level in Unity as opposed to preview specific settings so do send me off to read relevant documentation on the matter if you think it's something I should be able to figure out!

I'm having trouble setting up a grid that I can repeat tiles in that match up perfectly. Whether this is down to the fact it has to be pixel perfect or not I'm not sure - the assets I have are pixel art but the tiles don't seem to line up correctly - there may be small gaps or visible edges around each tile.

Am I right in saying that if my Pixels Per Unit is set to 100 on a sprite where the image is 512px * 256px then the tile map grid cell size should be 5.12 x 2.56? Note, this is all isometric. Could it be down to the fact it's pixel art and there's additional setup needed to make exact pixels line up?

Thanks for any advice