So tonight I was at the gym and the episode of Buffy I was watching was one I never really liked so my mind was wandering and then I remembered the pink pavement quilt. And how it was sitting, unbacked on one of my shelves. And I couldn't quite remember - was that 120cm wide? Could I try out my new backing for that?
Turns out, the answer is no. I made it be 135cm side, and repeated maths were stating quite insistently that the widest my backing could become was 145cm, and while 5cm is technically the width I trim my fold-over-bindings down to (I like a wide binding) I wasn't positive that it would still be 5cm after I'd quilted it all, so it looks like I might still need to make a new quilt top anyways.
What I can show you however is how you do it. You need 210cm of 112cm wide fabric, with about 6cm of selvedge cut off, since you need to do both sides. This is shown here as a piece of paper 21x11cm, which is why the points of the diagonal aren't right on the corners. I haven't trimmed that imaginary selvedge off.
You cut each of those 45 degree lines and shift the triangles to the other side. They don't have to change direction or anything, just put the top one at the bottom and the bottom one at the top.
It's pretty much the lamest tangram ever. It's now a 145cm square though, and that's the important bit. And all your seams are at a 45 degree angle to the ones on the front, meaning you don't have to match anything up when you're quilting. The only bad thing would be that the points on the corners will make it bulky at the part where it's already got extra layers at a fold-over binding, but given the amount of trimming I do on my corners anyway, I think it'd turn out fine.
I'm bummed I'm not gunna get to try it properly for a bit, but I know better than to try and make anything non-urgent in the week before a BnS.