Today I finally got a thing off my bucket list. 12"/154mm. A gain ratio of just barely less than 1.
The given crank length is actually to the middle of this hole.
I guess the main problem with this setup is… That it has no handlebars!
Fixed! Ok. So now let’s ride this machine. I won’t merely struggle with pedal strike, but actual crank strike so my strategy is to pull the saddle for each half rev to drag the uni after me.
Well, this was even worse than I expected. Even after a few attempts I just couldn’t make it through the second half-rev. The foot just slipped of the pedal. I would need to keep up the speed or somehow attach my foot to the pedal. Also I don’t want to ruin my son’s hub so I left it at that. @Ruari suggested I find a thin, elevated curb, but I will leave that challenge to someone else.
Also, this was so difficult to pull off that I am considering to remove another thing from my bucket list:
Riding a spokeless hub with only frame, cranks and pedals. I might still attempt it though, @Wheelou 
Maybe that is the future of unicycling - 0 gain ratio!