noted there are many mistakes and recalculated it - so best to be aware of it. The issues ate mentioned in the comments here: https://www.yellowbugquilts.com/mondrian-inspired-quilt-pattern/
I think the problem with the maths is that the sizes quoted are cut sizes, but the diagram makes it look as though they are finished sizes. I look forward to seeing the finished quilt - some lovely opportunities for trying out all sorts of different quilting patterns.
You're a proper darksider now @Manuela