Learning Rate (Log rule)

Hi , I need help with the Log rule. Can you help me with the following question : A manufacturing company wants to measure the learning rate that it has achieved with the manufacture of a new product item. The first item produced took 6 hours to make and the average time for the first 16 units was 3.3 hours. What has been the learning rate for this product, to the nearest 1 % ? I understand the doubling rule of 2 to the power of 4 gives you 16. So from the first of 6 hours and 16 of 3.3 hours I divide 3.3/6 to give me 0.55. so 0.55 equals R to the power of 4. From here I get slightly confused ... I calculate it as Log2(0.55) which gives me -0.862 so to the nearest 1% i say its 86%. Can you advise me of a slightly different method ? or how you would tackle this question? Thank you in advance :)