Using a crappy old pipe cutter is a good way of wasting expensive copper pipe. I found one in a box of plumbing supplies from my father. As long as I maintained significant sideways pressure during the first couple of rotations, it tracks true. Otherwise it travels a spiral. With only about 20cm of waste, I got the pipes I need. I’m guessing a bit on the exposed extension that will work, based on space available but with no idea about how I’ll run the pipes back to the manifold.
In any case, the pipes within pipes are ready for the LED ribbons, which I also cut to length. I purchased the waterproof ribbons this time, but they are stiffer due to the silicone coating and won’t conform to the copper pipes as well as the uncoated ones I’ve used in the past. I’ll think about it and have a go tomorrow.