This is not a how-to but more of an FYI -

I found a cheap accurate pressure sending unit on eBay for $30 shipped (pigtail included!), I verified its output with several known sources (air compressor & gauge, and mechanical trans pressure gauge) and it works a treat. The seller is "renbusan" and the eBay store is named "Industrial-stuffs". The sending unit I used is labeled "500-psi", is accurate to +-2%, runs on a +5-volt source, and has a third wire which is the sensor voltage output which can easily be monitored as a Flashscan analog input, the formula for the custom PID = "iff({EXT.AD3} > 0.4, ({EXT.AD3} - 0.4) / 0.0077, 0)".

Mr. P.

PS - the same seller has a 100-psi sending unit that would also work well for monitoring fuel pressure, for $30-shipped - this is by far the cheapest option I have seen for a fuel pressure sending unit, although I don't know what the PID for it would be (you would have to determine that with testing). The next cheapest fuel pressure sending unit that I know of is the $65 one sold by Aeroforce.