Iconoclast -- RPG -- The Rule of 100
Every attempt to do anything is based on a percentage chance out of a whole. Thus, since everything you might try to do is based on a 1 to 100 percent chance, every action roll is based on a d100. You will never need other dice. They're neat to have around, but they merely do what a d100 does, only less successfully. Whether it's a d6, a d8 or whatever, that die is just another way of carving up a whole into equal parts.
100-sided dice do exist, but we highly suggest you use 2 10-sided dice to determine the d100 roll. One die represents ones and the other represents 10s. Together, they generate a number between 1 and 100 (00). Having them be of a different color is always a bonus--the lighter/transparent die should represent tens, and the darker/opaque die should represent hundreds.
Anywhere in this system, 1 point always equals 1 point. There's no silly conversion or lookup charts or advanced calculus required. 100 equals a 100% roll, 50 equals 50%, and 1 equals 1%. One thing worth nothing, however, is that there is no way to "roll" a "0." Since any attempt to do anything offers at least a slim chance of success, inaction is the only "0" chance. However, this does not mean that a roll of 1 is the "worst" possible roll; in fact, a roll of "1" will almost always succeed, albeit barely.