Mate, get yourself some 1mm spacers and a pack of tile wedges
Set out as per normal and then start tiling using your spacers, every so often put your spirit level across the top of the tiles and make sure it's all plum and that all the topps of tiles are touching the bottom of your spirit level. Where you notice slight differences in width of grout line (or where the top of your tile does not touch the bottom of your spirit level) adjust accordingly using your wedges to find a happy medium.
Different sized tiles are a real pain so Sometimes you just have to find a compromise so that all the grout lines look as near as possible and this is where the wedges are so usefull as you can push them in and out accordingly.
Good luck :wink_smile: