I'm painting my daughter's room and want to do a bunch of different coloured circles behind her bed, crossing from one wall to another. I'm using a pencil and a piece of string to draw them. This is working fine for the ones that are entirely on one wall but I'd like to do one that crosses onto the wall that's at right angles with it. And for the majority of it to be on one wall, with a smaller section on another. How on earth do I do this? I've googled and can't find anything. There must be some sort of maths that tells me how long the string should be, or where to position it but I can't figure it out. Any help would be hugely appreciated.