Thanks to @Troy - 7% of existing owners have reservations as of the Q1 earnings call
This is going to sound a little tricky but that is not actually what Tesla says here. They said 7% of reservation holders are Tesla owners, not 7% of owners are reservation holders. The second percentage is much higher and it is possible to calculate it. Here is my attempt:
Tesla made that statement on 4 May 2016. At that time, the Model 3 reservation count was 348,529 units (after the cleanup removing cancellations and duplicates). 7% would be 24,397 units. On the same day, the global Tesla fleet was 129,980 units. Therefore 24,397/129,980= 18.8% of existing Tesla owners reserved a Model 3.