I'm pretty sure Neville's setup is correct. The Pressure sender is probably gauge pressure, i.e. Absolute fuel pressure - Ambient pressure. and the regulator regulates relative to manifold pressure. So to read the pressure that your regulator is actually regulating, you need to add the ambient pressure back in, and then subtract off the manifold pressure to get a differential pressure between absolute fuel pressure and manifold pressure.

It looks to me like your display is showing the gauge pressure.
-Before start, your MAP is as high as it can be (ambient). Since your regulator maintains some specific setpoint above that, it too is at its highest value.
-After start and at idle, your MAP is about as low as it can be, so you see a huge drop as the regulator does its job.
-Fuel pressure increases again as you increase power because the regulator is maintaining pressure relative to MAP.

If you can make your G3X do what neville's AFS is doing, you should see pretty constant fuel pressure at any power setting and altitude. An "offset" to me sounds like a constant bias, and not what you want, but I'm completely unfamiliar with the G3X.