You should use 2-core cable, 1.5mm thickness, for your starter-leads. This will allow you to position your LED strips up to 15m away from the transformer.
Follow these wiring guidelines, and the brightness of your LED strip lights will be consistent from start to finish.
How would I install LED strip lights in a rectangle?
Are you planning to install LED strip lights all around your living room, in a rectangle? Are you wondering how best to do this, and how many cables you’ll need? Well, here’s a wiring diagram that will help.