I think I wasn't clear. I had the owner install the EDM 8 or 9 years ago. It was most instructive! Before then, I could get it to what I knew was LOP by some amount based on the single point EGT but I had no way of knowing if all cylinders were sufficiently LOP. Also CHT temps were a factor. I can easily see with the EDM whether or not I'm sufficiently LOP. Can't do that with the single point. Based on EDM info I'm pretty confident that sans EDM cyl 3 & 4 on each engine weren't sufficiently LOP. 50 deg LOP on the single point isn't 50 deg LOP for the richest cylinders!
Fuel flow to 50 deg LOP on the single point is about .7-1.0 USG greater than every cylinder at least 50 deg LOP on the EDM.
As an aside, that EDM paid for itself within the 1st year of installation. The plane had a cracked LH #2 injector line that showed in odd temps on the EDM. I could see the temps going awry compared to the other 5 cylinders, even as I enrichened to compensate. I was thinking a return to base might be on the cards. A few minutes later it broke completely (not that I knew the cause at the time, but I could see it was a single cylinder issue) and I shut the engine down.
Because I knew it was a single cylinder problem, and there wasn't a fire, I elected to return to base - saving a lot of $$$ in transport for the pax & myself, finding a local mechanic to fix it, fetching the plane afterwards etc. I roughly estimate that those costs would have been about the purchase price of the EDM. Add in the fuel savings and it's been a no-brainer for the last 8 years or so.