The Lie
Yes, I'm sorry, but I lied to you. Faced with a choice of an explanation that was merely almost unintelligible or one that was totally unintellible, I chose the former. The truth is, that the actual calculated sewn base line of the triangle is not exactly 1" less than than the raw base line, as I indicated. The raw line is 5 1/4", as stated. The actual base seam line, or sewn base line, rather than being the 4 1/4" I used for my discussion, is actually closer to 4 3/8". This made for very awkward numbers to be chasing around. When I measured my sewn sample, I got 4 1/4". This was much easier for me to work with. I figured that with the normal variations in seam guides and piecing feet, plus such issues as differences in pencil point widths when drawing lines, getting too exact wasn't really that important. If, in fact, that difference produces a slightly larger quilt in the horizontal dimension than my calculations came up with, it won't by by that much and does it really matter? However, there is always an engineer hiding somewhere, so I'm including this special explanation. Sweet dreams!
Captain Dick