Here's the formula so you can make the size you need:
Unfinished size of unit + 1/2" x 2
Cut 2 squares that size..from different fabrics, of course!
Example : for a 4 x 4 units 12" block (finished) you need 3½" unfinished units (includes the seam allowance)
3½" + ½" = 4" x 2 = 8" squares to cut to get 8 HST units.
On the lighter square, draw a big X from corner to corner:
|Use a pencil (not pen) and make a line you can see. This will be a cutting line later.|
With Right Sides together, sew 1/4" away from both sides of both lines:
|Stitched in red thread for visibility!|
Cut right through the middle of the block, but do not move any pieces!
|Be sure the fabric is under the ruler...edge of ruler through middle of the X....line up across any marks at the bottom.|
This is just cutting, not using marks to measure anything.
Cut through the middle going the other direction:
|Working at the corner of the table, you can just move to the other side.|
In case you want a better visual, you just cut where the green lines are in this pic...but don't bother to draw them, just use your ruler to cut....
|After these two long cuts, it's OK to move the pieces.|
Now you can cut on the lines you drew:
|Now it looks like the usual set up for quick-piecing HSTs|
Press the units open (usually toward the darker fabric).
They should be the correct size. If not, trim them to equal the size you need.
That was fast and easy...with minimal math, all the units can be used, and the bias edges are never on the outside.
If you have some pre-cut squares, do Reversed Math (divide by 2, then minus 1/2" for the unfinished size...minus another 1/2" for the finished unit size)
5" squares = 1½" (2½ -½" = 2" unfinished size, for 1½" sewn in) tiny!!!
10" squares = 4½" ( 5" - ½" = 4 ½" unfinished, 4" sewn in) another 12" block standard
OK, that's more "math" than I can take in one day.
And to you Math Majors... I do acknowledge this is actually arithmetic! (Vocabulary I know...numbers, not so much!)
Another disclaimer: (Oh, I know there are a few sharp-eyed readers...)
the demo block squares were cut 12", leaving a rather odd size of 5½" units, but this was created for a large group demo and needed to be seen from afar!