Grout width to match different size wall and floor tiles

[DrSeuss]

We've decided on a grid layout for the tiles in our new bathroom and I'm getting a bit stuck with setting out.

The wall tiles are a 200x200mm square tile and the floor tiles 100x100mm.

I want the wall and floor to line up without running out, but struggling to work out the grout width to achieve this.

Is there a formula for this?

 

[DrSeuss]

I got out the spreadsheet and realised that what you end up with is a floor tile that must have a grout width of 1/2 the larger wall tile.

This would give either a 2mm floor grout line and a 4mm wall grout line, or a 3mm & 6mm.

3mm is listed in the wiki as the recommended minimum for floor tiles, but 6mm seems really quite a large grout line on a wall?