RR Trent 800 idles at around 19-20% N1.
But this does not mean that the engine is producing a fifth of its rated thrust at idle.
% for N1,2,3 is only an indication of blade speed. I remember reading somewhere in the AFM that N1 can achieve about 2-3,000 rev per minute. So a reading of 20% N1 means that the blades are doing 400-600 rev per minutes.