That nut's a bugger. I use a similar method where I braized an M8 nut (same as cylinder base nut) onto a 12mm spanner just long enough to poke out into the breeze far enough for me to get my torque wrench on.

You do have to compensate for this extra leverage and adjust your torque wrench accordingly. To be sure I then use the same "extra reach" tool for all the nuts just to make sure that the torque is identical.

Then I wised up and used the proper Honda tool, which works only if the RC valve cables and gubbins are removed. This allows direct fitting onto the nut.

This tool won't work on the 300 cylinders 'cos the casting's in the way.