Pretty block! When cutting out your pieces, the line on the ruler should be used to measure not the mat. The lines on the ruler also should be on the fabric cut edge not over it for that tiny bit of extra fabric in your pieces. Test by sewing two squares together and measuring the finished unit to see if you are getting an accurate seam allowance. Cut your pieces on the straight of grain whenever possible. Pin any bias edges so they don't stretch as you sew.