It turns out, there's a lot of rainbow stripes in my stash - two thirds of this bundle were just sitting there waiting, and there were 6 more that got taken out and replaced for a variety of reasons.
I'm thinking of doing the Lattice quilt with rainbow stripes and a blue background, just because I'd like to see what it would look like. I'm also thinking of doing it with 4 repeating solids, on a very dark grey, but I'm very slowly writing up a list of solids that I need next time I do a fabric shack order, and frankly, it's not urgent. The light blue of the background is a Prima from Spotlight, and it's a near perfect match to Kona Aqua.
Oh, and if you thought yesterday's maths was hard to understand, this is how it was actually worked out - on the bottom of a cardboard jewellery box, 2 square inches, because I couldn't find any paper handy.