Spring seated check ball used in a low volume low flow (130 cubic in./min max) hydraulic valve device.
The device uses a .250" steel ball, 4.4 lb spring with a 54 lb/in spring rate, 118 degree included angle seat with a .0147 sq. in. contact area.
The original design intent was to prevent the actuation of the device until a pressure of 300 psi has been reached.
What cracking pressure did you design the spring and ball combination for?  I.e. how much preload is in the spring when the ball is seated?
If your seat area is 0.015 in^2, or roughly a .14 inch diameter circle, and you put 4.4 lbs. of preload on the ball, then the cracking pressure should be roughly 300 psi.  That sounds fairly close to your number of 400 psi.  This is for a very low flow rate, when the ball has barely lifted from the seat.
With no spring or ball, the same orifice should produce a pressure drop of about 1 psi (using water as the reference fluid).  I'm assuming there is little or no restriction between the ball and the valve housing.
The ball and orifice together with a 54 lb/in spring should produce a bit more drop.  I usually use 20-30% of seat diameter as a rough opening distance for the ball, which gives a spring deflection of 0.2*.14 = .03 inch, or spring delta-force of .03*54 = 1.4 lbs, acting on the projected seat area of .015in^2 gives an additional 100 psi.  This is an upper ballpark for the flowing check valve.

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