One word of advice in using the marks on your cutting mat.
check to make sure they are accurate and also be careful and always line up on the same side of the mark. Some mats have wider lines and can throw off your accuracy.
I use them when I am not too concerned about a thread or two off. Same is true of using marks on rulers - all are not the same so consistency is important. I took a class from a well known teacher who teaches accuracy and she told us to aways cut on the outside of the lines and that gave a just a bit more and compensated for the scant quarter inch in piecing. It works