Dinner is over and now I'm posting how I do my hst. At least, this is how I do them when it's just one lone block, most of the time. I have Thangles, Triangles on a Roll, and Triangulations 2.0, and have used all 3, depending on what's closest to hand at a given time. I probably do this more than anything, though. I hate tearing off paper, even though I know all about using a small stitch and snapping it. It feels more time consuming to me. If I'm doing a whole quilt of something, though, I usually go that route.
Barbara said 2-7/8" and that is mathematically correct. If you can sew a perfect seam every time, and everything always comes out square, go for it. I cut these at 3" and probably would have been less stressed at 3-1/8". For today's eight-inch block, here's what I cut, light, medium, dark:
It seems too time consuming to draw diagonal lines when I can just press one of the squares diagonally down the middle, very carefully going corner to corner.
The fold line is the CUTTING line. I sew 1/4" on each side of the fold, and I find it easier to sew if I've pressed with right sides showing and wrong sides together. The other way works, too. Then I set the seams. Someone asked me what that means. When you stitch it, it isn't completely flat, so you press it and it looks much nicer and sets the fabric. Press. Don't slide the iron around all over the place. Then I cut on the diagonal.
After pressing toward the dark, I trim my squares. I line the 45 degree line of the square ruler up against the diagonal seam. It should line up all along the line. I needed 2.5" squares, so I made sure that I was taking a bit off these two sides, then turned it to take some off the other two. This insures absolute 90 degree angle corners, and straight sides. It also cuts of the "ears" on the corners.
It might help you to put colored tape along the lines of the 2.5" square on the ruler. There is that clingy stuff that isn't really tape. I don't do this, but I've been doing it for years. And I've been known to make the wrong things line up now and then.
Does my method waste a little fabric? Yes, it does. Here's how much I wasted in order to have perfectly square, flat 2.5" hst for this block:
Next, I laid out the hst in the order they should be sewn. I sewed the down sections, then sewed across. I did not press in between those two stitchings. Things lined up very nicely.