Quote from N1 Vibes:
The relationship of N1/thrust is only as accurate as the day the engine comes off the test-cell, and the N1 trim is set. When the fan track wears out, the airfoils get dirty and the blade l/e's erode etc the N1 speed is no longer related to the thrust it was in the test cell - therefore on a high time CFM engine you may be 2% down on thrust.
[Unquote]
Point taken. [And maybe the N1 VIB off-scale at 9.9(+) means one of the fan blades is missing anyway?]
Trouble is, if memory serves, there's something non-user-friendly about EPR readings, particularly at the medium and lower power settings; like in the hold and on instrument approaches. But I can't quite put my finger on it. This was particularly true on the JT3D and the later Speys, both of which had analogue gauges where you'd struggle to see the difference between 1.20 and 1.25 (= quite a power difference).
Perhaps someone like Mäx Reverse can remind us how many places of decimals are displayed on the presumably accurate, digital V2500 EPR display. Is it 2, or 3? If so, maybe the trick I never learned might be to knock off the "1" mentally, and concentrate on the decimals. Then come up with some rules of thumb as-per N1.
But for those who fly both engines, like BA and currently easyJet, it's going to be a challenge to remember both sets of figures. Particularly if the management frown on (or ban) the use of manual thrust anyway.
Here I go again...
