Simple math. That's why the tilde.
Averages between 3bar and 4bar are about 1.15 across the board, comparing other known HP ratings for 3 and 4bar outputs...
at 1.14, you get 404whp. at 1.16 you get 412whp.
All I did was divide a known 4bar whp rating from a known 3bar whp rating for the same injector.