Ok, after going back and forth on this, here is my untrained analysis of the information I could cull on this subject. On my IO-550N fuel pressure can be measured both metered and unmetered. Unmetered fuel pressure measured between the fuel pump and fuel servo will show pressure supplied to the engine or fuel pump output. Metered pressure measured from the flow divider will show pressure wanted by the engine and will vary with throttle position.

It would seem the interest here is that the supply pump is working properly, a situation that could be clouded by the variability in pressure caused by throttle and mixture position if sensed at the flow divider.  A low or high unmetered fuel pressure would indicate the engine is not being supplied with enough or too much fuel and this seems like the more critical information.  The gauge port on the flow divider for metered fuel pressure seems to be used primarily to drive a pressure type fuel flow gauge not applicable in my case.

I will be re-plumbing to measure unmetered fuel pressure as this is what Garmin values are set to on the G900 EIS for Continentals (Garmin measures metered fuel pressure on Lycoming engines).  I hope this makes sense.

Jon Socolof