There is no Magic Threshold.
Taxation isn't theft, because governments were always endowed with some sort of rights over your property, perhaps bound by law as to the extent to which they can appropriate it.
To commit theft a government would have to appropriate property over which they have absolutely no rights, or beyond a law-determined threshold.
I'm no so sure if this is an appropriate argument, when property is determined by law.
While I try to avoid discussions of morality (save when debating moral systems directly) there are several cases where it stands to reason that collectives be endowed with more rights than individuals.
E.g. in a poor country where each villager commits $1 to a government to crowdfund national security - which they individually would not be able to afford.
Though an implication may be that this collective is also endowed with the right to kill, that individuals do not have.