Not the right kind of spacers. You don’t need a leveling system with tile that small. You just need the right kind of heavy bodied mortar. And experience. Herringbone is for experienced tilers only. Herringbone with variable thickness chinese slate is best of the best tilers only.

Rachel - what did you wind up doing for your slate floors re: grout color and width size? I am not installing our tile but I want to figure out which size grout line looks best (obviously, I will listen to the professional I’ve engaged - but there will be some allowable width variances). My tile is not Brazilian slate (I believe it’s from Vermont) - mine is more grey than black (as yours appears) - definitely not a blue/green color. I just really would like to see the size of the line and the amount of contrast you went with with your tile. If you wouldn’t mind letting me know + posting some photos of your finished project, I’d appreciate it! Thanks!