The dovetail on the P09 is .070 deep
So take .070 + your .1615 = .2315 which equals 5.8mm
The 6.5mm front sight from CZ custom on my P09 measures .185 from the top of the slide.
As I said before, you have to KNOW how they are measuring the sights, some places measure total height some measure blade height, two different measurements.
The smartest and easiest thing to do is, install the front sight that you plan/want to run. Shoot the gun, how does it shoot. You might not even be able to tell the difference between a 5.5 and a 5.75 sight. If it doesnt shoot to the POA that you prefer, call up Dawson Precision and they will make you a custom rear sight to correct the issue.
No point in worrying about changing rear sights and this front sight was supposed be this measurement but its not til you install the front sight and confirm the POA/POI.