For good DeClipping only one thing is needed, good input tilt. I think ST is only program that has Input Tilt adjustment (standoalone), wich is ideal for analog inputs. DeClipper internaly also looks for already clipped and then tilted tracks up to 45deg by default.
How we could adjust the input of st ? Like tilting on fm output? By producing square wave of 60hz and trying to optimize with rc correction of input?
Input tilt, just play 30-60Hz Squere tone from your usuall source/player/playout, go to ST and watch input scope.. horizontal lines should be 100% horizontal. And this is only if your ST input is Analog. With digital/network/etc input should be already ideal, i anyway always check.
Output FM Tilt is same but bit tricky, You need to have really good tool to measure that squere tone from your transmitter, from AIR. Erlier I used my Kenwood tuner with MPX output to my USB 192kHz soundcard. All that calibrated with Rohde-Schwarz Unit - Tilt and MPX level.
Now it's waaay easier for me since u use MPXtool with SDR.
You can also do it with Pira.. play squere tone and tune Output Tilt so MPX output level need to be lowest as possible, meaning horizontal line is ~100% horizontal