I believe it is a well documented limitation of FSX and other developments have also used Corrected N1 as the basis for generating EPR values.  My modified guages work quite well based on the data I have. It may be a little off at very low values -- less than 1.3 as I could not fit a curve to that part of the data.  Also, the relationship between N1 and EPR is going to vary a bit with engine variants. I believe the data I have used is for one of the earlier engine types. So you need the performance charts for the same data, which is readily available to download from another developer that did an FS9 727.

I don't want to annoy Captain Sim, so I guess I'll just post the code and how to use it on the un-official CS forum on Avsim within the next day or so, when I have some time.

The other thing I really would like to fix is the inability to descend at engine idle at a realistic speed, although I think that might be quite difficult to do without messing up the rest of the flight model.