Those chuffing half rectangle triangles

I thought I had conquered the half rectangle triangle but it has reared it's ugly head once more! This looks OK doesn't it, seam allowance wise? It's not!Margaret on the Popular Patchwork forum was having trouble scaling up these rectangles from a pattern, which I thought was quite simple at first having followed this tutorial from The Modern Quilt Guild HERE where the the fabrics are pivoted on the diagonal to produce two blocks at a time. I even used this technique for my Wallander pattern which I think I'm going to have to pull for the time being whilst I make it up and fiddle with these until it works.The problem occurs when joining these blocks together as I will demonstrate. I'm after a finished size of 9" x 4" (but this will not be the case as I shall show you!) so I cut rectangles 10" x 5".I originally misunderstood and sewed down my marked line. No matter as I now have sewn either side and cut down the original stitch line then snipped off the dog ears...No not his, he has taken a liking to the Around the Bend bee top of Brittany's that these blocks will be for. Even though it is heavily cat based.

So I stitched up four of these and the problem Margaret had made itself apparent. It's been a while since I did geometry, but this is how I understand it. The angle of the diagonal will change depending on the dimensions of the rectangle. The angle will be more acute for a long rectangle than a stubby one. On a square the angle is 45 degrees, splitting the 90 degree right angle exactly in half so it will reduce equally on all sides when seam allowances are taken off. The diagonal will always end in the corner if you are strict with keeping your seam allowances the same, whether they be 1/4" or 1/2". With rectangles it varies and so this doesn't happen. See?I have a much bigger than 1/4" seam allowance on the top and bottom and none at all to play with on the left and right. I found a way of fixing the sides to give me more room but it's a bodge. I used a huge 3/4" seam horizontally to achieve this (and I imagine 1/2" seam allowance will fix the top or the bottom):Still, this means my supposedly 18" x 8" diamond is realistically 17" x 7" which has ruined the size I needed. Does anyone know if there is a ratio that this will work for or have any tips? I'm going to continue to have a think in the meantime. I reckon paper piecing could work but I could do with a way of doing it without as my Wallander quilt will use about a ream otherwise!I'm going to have a Baileys now, I earned it.