Henry should be allowed to keep 10 Blibs each day. This will allow Frank to survive and buy a new Blib machine every ten days, while allowing Henry to keep the fruits of his labour that are not required for Frank's continued survival/ownership of a Blib machine.
If I were Frank and I didn't have a conscience, I would only let Henry keep 4 Blibs per day. This would mean that he could not buy his own Blib machine, and he would keep having to operate mine. Fortunately, I do have a conscience, and would allow him to keep 10 Blibs per day.