How fast did Dr Ryland Grace travel in the Hail Mary Project?

[Twinduck]

For those of you who haven't read the book, it is a science fiction story in which a man named Dr Ryland Grace travels to Tau Ceti. The star is approximately 12 light years from Earth, and he got there in about 13 years, which for him was a 3 year journey. He is accelerating at 1,5g constantly for half the journey, then decelerating at 1.5g for the other half.

What percentage of the speed of light would he reach after accelerating at 1.5g for 6 and a half years?

 

[Nugatory]

What do you think and why? Googling for “relativistic acceleration” will find some useful formulas.