Sounds like you're making half-square triangles?? I would cut my squares at 9" and then trim the finished HST's to the proper size.
But back to your original question...if you're cutting at a 7/8" marking it would generally be the small line just prior to the next inch line. Provided your ruler/mat is marked with 1/8" lines. If there are 8 smaller lines between the 2 inch marks, you are good.