I have No.18 fitted to all my locos, 121, 141, 181, 201, & A Class, 27 locos in total. The back of the buckeye head needs to be in line with the buffers. You could use No.19 if you wish, No.17 too short, No.20 too long. As previously said track curvature also plays a part, add to that coaching stock varies as well.