Sorry if this is info you already have. Some info may not fit your gun or circumstances but it's a good read if nothing else.
Well to start this right...Every manufacture has built there spray gun to work best and give you the smoothest finish at 10 PSI Cap pressure.
Now at 10 PSI cap pressure you will have 18-23 psi at the gun with the LPH 400. This does not mean some will not get better results with Higher pressure because the Speed and distance at which you spray can affect the finish.
people who hold the gun farther away or closer than the 4-6" recommended will use a different pressure to get the Same results. Also user speed can affect the way a gun sprays...You should remember that on an Iwata 10 Psi cap pressure will Always come with only 18-23 Psi at the gun...But then this is with Using a "Quality gauge"...At 23 Psi some gauges will read 24-29 Psi
Air hose length and Diameter all come into play also...as well as Available compressor volume..... So bottom Line a Cap Pressure gauge is the only way you can be for sure of the required 10PSI at the cap
Any more Inlet pressure causes Higher cap pressure which is Illegal and just causing more over sprayand wasted paint...So as you see...testing the pressure at the cap is the only way you can make for SURE of the cap pressure..CAP PRESSURE GAUGE
And don't forget the Compressor Volume test....Spray Gun Volume control wide open..with no fluid in gun...trigger pulled all the way back..hold for TWO minutes..if the Compressor will not hold the required cap pressure for two minutes then you compressor is too small..some try to Cheat by turning up the pressure...may not always work
Bottom line is you need to practice with your Spray gun whatever the brand to find the best pressure for you....taking into account all of the above.
Find the Solution that fits your needs
Here's some more good info.
https://www.youtube.com/watch?v=HDuuJvyiZiI
Again, hope something here can be of some use. G