OK - so the answer is, the number is really really small.
Close enough? :~D
Close.... but I think you misplaced a decimal in this calculation:
2,700,000,000 / 16,000,000 = 0.59%
Should be 5.9%....
but, according to the actual report from MS, those ~16 million malware were only on the 5.7 million infected computers. The reason the number of malware was greater than the number of infected computers was because they found that if a computer was infected, it was likely infected with more than one malware.
So the ~16 Million malware were actually on only ~2.1% of the computers.