In addition to the fact that E = pc, what two assumptions did Einstein use to derive his famous equation E = mc2? Below what fraction of the speed of light is it generally acceptable to use the...

In addition to the fact that E = pc, what two assumptions did Einstein use to derive his famous equation E = mc2?

Below what fraction of the speed of light is it generally acceptable to use the expression for the rest mass of a particle without making an error in the calculation of its mass or its momentum?