The pie-chart math looks accurate to me. It said in the book that killing tears a soul in half. He would have begun with a whole soul and after he made the first Horcrux he would only have half a soul. The other half would be the Horcrux in the diary. So he would then split his 50% of a soul in half again when he made the Horcrux in the ring. 25% would go in the ring with only 25% left for himself (50% of the 50% he had left is 25%). And this would go on until there was 0.7% going to Nagini on the last Horcrux and 0.7% remaining for himself. A very weird thing to think that his first Horcrux had 50% of a full soul, while he was left with only 0.7% after that last split with Nagini.