Reinventing the wheel (or at least the HRT)

I indulged in a few long afternoons of playing in EQ8 recently.  As usual, what I ended up with bears no resemblance to what I started with. This instance was rather extreme: I was thinking flowers and somehow ended up with a sailboat.  Go figure!

This block design ended up in my Sailing School pattern.  
For a tutorial about HRT, go here.

After I drew it, I printed out the cut sizes the software suggested and happily started cutting and sewing, only to find that my sails were not turning out the size they should.  I suppose I could have winged it and fudged all the other block parts to make it all fit together, but I'm really not an improv kind of quilter.  I can appreciate a well-made improv quilt, but I don't particularly enjoy the process myself.  I like to know what to cut to what size to make things fit in a predetermined way.  So there.

This of course meant that I needed to figure out why things weren't working out. I double- and triple-checked my seam allowance.  I cut more fabric, checking my measurements.  Nope, still not working.  I finally figured out that my problem arose from having to guess how to line up the triangles when I sewed them together. I wasn't hitting the sweet spot.  I thought I knew where to align one piece relative to the seam allowance on the other. I was wrong, though it would have worked if the geometry of that long triangle hadn't messed up the works.

To make a long story short, in a HRT unit, if you want the diagonal seam to reach perfectly from corner to corner, you don't want the seam to be exactly on the diagonal of the unfinished unit.  I drew it out on paper to wrap my mind around it. The paper is cut to the unfinished size of the unit and the drawn rectangle inside is the finished size.  If you extend the line that runs diagonally from corner to corner in the finished size, you see that it does not actually cross the corners of the unfinished unit.

Well, that's a fine how-do-you-do! I know there are rulers out there that take all this into account.  In fact, I have one on my wall that would suit, except that it isn't large enough for the unit I needed.  Also, the quilt design I have in mind uses just a few boat blocks, and I'd hate to ask folks to buy a specialty ruler for just a few blocks.

I humphed and harrumphed and decided there must be  away to make these a little larger and cut down to size so alignment wouldn't be such a bother.  I took in ideas from various tutorials, and spent half the afternoon stirring them all together and finally came up with something that worked the way I wanted.  

I spent the rest of the afternoon figuring out the math to know what size rectangles to start with to end up with a given size.  It would be a pain to use trial and error every time I want a different sized unit.

There's a tutorial in the works to share the details.  In the meantime, tell me, have you used HRT in any projects?  If so, do you have any tips or trick to share?

UPDATE:  The tutorial is now available here!