Perhaps I'm missing something but when I make half-square triangles, I draw the cutting line on with pencil. Then, I sew the first pass with the right edge of my 1/4" quilting foot against the drawn line. For the second pass, I turn the piece around and again sew with the right edge of my quilting foot against the drawn line. The two seams are sewn in opposite directions.
When I check that with the turn of the cloth my block will be the correct finished size, I may find it necessary to run the quilting foot exactly down the middle of the pencil line or slightly to one side or the other. Another option can be to adjust the needle position 1 click to the left or right [check that you can do that by turning the handwheel first without the needle hitting the side of the presser foot].
The other gadget/notion that I use is the "Angler II" which means that I don't have to draw any lines. It's available in Canada from Joanne's Creative Notions [not related to Joann's in the US] http://joannescreativenotions.com/store/advanced_search_result.php?keywords=Angler&osCsid=920be42b121c4ca8ddc3944078eb52b2&search_in_description=1
I've ordered several times from this company without any problem.
When you're going to be sewing so many half-square triangle blocks, you might be interested in having a look at "Quilters Academy Volume 3" http://www.ctpub.com/productdetails.cfm?SKU=10698
. It is all about sewing half square triangles -- a dozen different methods or more. In Canada, you can get it from either Chapters or Amazon.ca.
Remember that the method that the quilt pattern designer gave for making the half-square triangles is only one method and you may find that a different method works better for you.
In a quilt block like this one, it's important to be consistent in your block size. I would not cut any of the plain squares until I had the half square triangles done. I would cut them based on my finished block size [which might vary 1/8" or so from the design block size]. Some quilters would cut the squares/rectangles for the half square triangles over size and then square them up to the proper size after they had been pressed. Half square triangle blocks can be easily squared up.