There are charts available on the web that show N1, N2 and EGT for varying TAT values for JT8D engines. I used the data points for a TAT of 15'C, since Corrected N1 is supposed to show what N1 would be at that temperature.  I used the charts published by another developer for its FS9 727 for my data.  You can see my Avsim post to determine which one.  But I also found another set of charts at a site called "my cockpit" for a later engine variant for the 727-200 -- I haven't made use of that yet.  Finally there is also a 727 website (www.boeing-727.com) with engine data for many variants ; it only gives a few data points for each engine type.

I used a regression program to fit a curve to the data points.

I don't have the source documents used to create the charts mentioned above, but I believe the original authors had something authoritative.