Very pretty block. With HST, I always round up an 1/8". Then after pressing your HSTs carefully, trim them down to 3". I find that if I sew a bunch of individual sections (such as a HST) and trim them down to the perfect size, the block as a whole comes out much more accurate.
Count the lines between the 1" & 2" mark on your ruler. If there are seven short lines, each one of those is an 1/8". If you only have three, then your ruler only have 1/4" lines. Good Luck.