@ZeeOSix beat me to it, as I am just rejoining now. The resulting force vector calculation can get a bit involved here but you cannot ignore the leg area altogether. I would make an assumption that the correct spring rate was installed and then pick a method that approaches the advertised specification, +/-.going forward. Also, I would say as you stated, it is correct to assume that the poppet doesn't have to be far off the seat to flow near the max orifice value for the Dp to normalize. Fun engineering exercise.... ... .
The 25% was just a rough visual estimation. The initial opening pressure is more important and should be more accurate.
There are four vertical plastic supports that hook onto the top of the spring and connect to the valve plate that gets pushed open by oil pressure. The area they take up on the top of the valve plate is area that the oil pressure won't be acting on, so I subtracted that area. Ignoring the supports, the calculated pressure ends up being a bit lower, 14.8-16.9 psi instead of 17.4-20.2 psi.
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