I'am using 2mm joint with calibrated 610mmx405mm Jura Stone.
The tile has a 1mm chamfer on the edge and any wider would give a massive unsightly joint - IMHO.
For 300x300mm tiles like yours I'd go for 3mm, but tile sizing can be an issue so check first.
Joint size and grout colour with floor tiling can often determine the aesthetic picture of the completed job - so great care is required in making that choice.