Real granite tiles will be more consistent in size than porcelain tiles. If you have a center pattern, you lay this pattern first, then expand towards the edges. You should measure the real tile first, then base your design on that. Numbers read from the box are just nominal dimensions.
When you measure the tiles, use a stick with 1/32 resolution. Don't toss the ruler around like 007 suggested. It is not going to work and will lower your IQ significantly.