I've messing around with that line in the aam program making adjustments to a aircraft - default Learjet (FS2004).
I noticed the first 3 lines in line 1506 are labeled 20, 25 and 30. I'm not sure totally what they refer to but I increased the values above these points to try and achieve a higher idle thrust setting.
Default .0254, 0.0508, 0.07933. I pushed them all up to between 0.1 to 0.14 or so.

This wasn't to 'improve' or 'fix' the Learjet in anyway, only to test.

Results with default - idle thrust 34% N1 and FF182. Aircraft at 10-20 kts G/S when going back to idle thrust reduces speed eventually to a stop.

After adjustments - same N1 but fuel flow is now 260. And at the same ground speeds, reducing to idle and the aircraft maintains speeds. And will slightly accelerate low weights.

The same fuel flow I get on the modified air file equates to a N1 of 37% on the pre-modified air file . 3% more.

Is that a typical adjustment for the 1506 line? It seemed quite an increase of adjustments for only 3% N1 although it had the desired affect of increasing idle thrust to a perhaps more realistic number.

Also to note, I couldn't see any difference in fuel flow or N1 at cruise altitudes comparing both default and modified. Is that typical?