I'm going to jump in here because this is the sort of discussion that fascinates me. Science fiction calls this type of thought experiment world "Post-scarcity" which is a counterpoint to 1984 which was more of a regulated scarcity economy. My thought is that when the world goes "Post-scarcity" there will still be things that are scarce such as ocean-front property or awesome tickets seats to see a live performance. The things that will NOT be scarce are food, water, electricity, comfortable sleeping quarters, wireless network bandwidth, clean clothing, and advertizements on the video program platform du jour. This is by no means a correct list, but to answer the question, "what do you do when you only need 50% (or less) of the available people to actually work?" my answer would be to ensure that they have the minimum "Post-scarcity" list and that in their free time they aren't causing trouble. Since a lot of these people will cause trouble, though, the alternatives are to stick them in jail or make some kind of fulfilling occupation available to them. So yeah, certain jobs that robots replaced humans would revert back to the humans. The economics basically becomes a muddle at this point.
"If all those things could be provided to me without working, I wouldn't work." There is a degree of leisure activities that becomes available if you stop working. You'd have time to do more things in your newly found spare time - some free like running outside - some not free like playing a round of golf. So if you wanted to golf, you'd still need to work (i.e. earn money) so that you can trade with the golf course to reserve your tee-time (this example works because I think there can never be a high enough supply of golf courses to meet the demand is playing a round is free and people have infinite free time).
"If you make it too low, they will be unable to survive." I think the greatest threat is making it so low that they organize, rebel, and destroy the companies who shifted from human labor to robot labor. It is tough for me to imagine a scenario is a 1st world country where technological advancement leads to people who are displaced gracefully exiting the human race.