This doesn't seem to be working out...
 

I just started on a simple baby quilt pattern called "Traveling Southwest" that I found on FaveQuilts. It says to cut 8-1/2" squares of print and background, draw a diagonal line on one, layer a print and a background and sew 1/4" each side of the drawn line, cut, then put with a 7" square of print and one of background to form a sort of arrow pattern.

The dimensions don't work at all. If I had the "geometry gene" I could figure this out, but what is wrong here? It seems obvious that the 8-1/2" squares are too large, but I have never been able to 'see' how a measurement will turn out after taking some seams, cutting, etc.

do it with two pieces of paper, that's kalamaquilts math :)
A visual to work from really helps me.

You are right. The directions say to make HST's out of two 8 1/2" squares, cut in half diagonally (which would give you two blocks app. 7 3/4" square). You could then trim them down to 7 1/2" (which would yield 2 FINISHED 7" squares)

So maybe the writer got confused between the size of unfinished and finished squares. Just add the extra 1/2 " to the ones that are supposed to be cut 7" square. it should all work out then.

I read the pattern. Says to trim HST down to 7" ....which is the size of the squares you cut, right.....seemed like it made sense to me.....

It just seems like such a waste of fabric to trim it down 1/2" each way -- why not give the right measurement in the first place?

It's common to over cut half square triangles, then trim down, but this is probably a bit more than you need. If you haven't cut all the squares yet, I would do a couple of trials, one using an 8" square and one using a 7 1/2" and see which one you are most comfortable with.
You normally add about an inch to the finished size, which allows you to cut a sliver off to square it up. If you are supposed to trim to 7", the finished size is 6.5, so a 7.5" square would be a more common measurement to use.

Is the math to add 7/8" of an inch to the finished size?? She is probably wanting to have a little extra for maybe a new person to quilting. You have to realize the teacher has to deal with a variety of quilters from advance to beginners. She is just making sure that no one comes up short. Good Luck and have fun !!

I have learned the hard way to do one block before cutting all my fabric. Once I get it put together I can then see what needs to be corrected.

This is good advice. I have never been so far "off" before, and unfortunately did cut all the patterned pieces already. At least it doesn't have 1,000 pieces!

I know these ladies on the board can help you figure this out. I always have to ask my DH.

The members of the board did help me figure it out, and you can be sure I changed the measurement everywhere it appeared in the instructions -- it took me longer to cut them down than it did to sew them in the first place!

there is an online quilting store that sells a set of rulers that automatically figure the additional amount to cut on half square triangles and quarter square triangles. You just set your ruler up with the correct set of triangles you want to make and cut the triangles the exact measurement you want. The ruler adds the additional inches you need for the finished product. I've included the link. My squares come out perfect every time.
http://www.guidelines4quilting.com/

The pattern says it is a 13 inch finished block, which would have driven me bonkers from the get go LOL , but it does make the 7 inch square part make sense.