I found that the rate is pretty high from 0 to 40 vet. It is almost 80% when there is an event of 200% conversion increase and premium account. Wait for an event for the conversion.
From my experience, the conversion rate is related to the level of the sailor (the higher the worse) and the number of vet on the sailor (there is a drop in % to convert at around 40 and then 100, 170, 230, and 280).