I also have the mcfarlane vernier assembly. I love it.
Follow the recommended instructions for install so that you don't pull the throttle cable out of its sheath, this is a small but fixable nightmare. The vernier assembly uses a set of springs and loose fitted roller bearings and if you pull the throttle handle out during install the bearings and spring become disassembled. Finding them might be a challenge, but there is a Youtube video for the reassembly.
Just don't disassemble it during install like I did and you will be fine.