My current project, Viola, calls for a construction method that I think is a very good one. That would be making half square triangles (and other block units) slightly over sized, then trimming them down. These trimmed units are all so uniform in size that sewing them together is a precision lovers dream.
If you have been sewing for years, the following tip may be old news to you. However, if you are a newish quilter, what I am sharing could be helpful.
There are dozens of special rulers for squaring up blocks, but you don't have to get too fancy. I am using my Creative Grids 4 1/2 by 8 1/2 ruler. This is such a handy size ruler, and I find myself using it all the time.
Do you see the diagonal line on the ruler that shows the 45 degree angle? That is your magic guide for squaring up half square triangles.
Now don't be confused, but I flip my ruler upside down to do my trimming. I want the three inch ruler marks to be on all four sides of my unit, and the 45 degree line running from lower left to upper right, right along the seam. Position the ruler with the 45 degree mark right down the middle, and teensy amounts of excess fabric outside all the 3 inch ruler marks. Using my rotary cutter, I trim those little slivers of fabric on the right side and on the top.
I then flip the block around as you see in the photo, and line up my just trimmed edges along the 3 inch ruler lines on the left and on the bottom of the half square triangle. Make sure that 45 degree line is still right on the seam. Then I trim off the slivers of fabric on the right and on the top of the unit again.
Here you can see the itty bitty slivers of fabric I trimmed off the half square triangle unit. What I am left with is a perfectly square 3 inch block unit. Nothing wonky here. See how that seam line runs dead center through the block, corner to corner? That's what you want.
It is surprising how those little slivers add up. This extra step takes time, but the perfectly equal sized block units are so worth it.