First thought is that it must be 10ft divided by the circumference of the earth (i.e. a very small number). However, my second thought is that this would have all kinds of bells and sirens going off on QI, so probably not that.
My third thought is that pi is involved, in which case this is probably too mathematical for me.
My fourth thought is that perhaps the answer is 10ft (for reasons I can't articulate), so that's my guess