It all depends on how relative or precise you need to be. Most people make their own ICC profiles for soft proofing. We supply them when we have them.
But, making a custom K6 or K7 curve for your setup will produce the same output. You would not be able to detect the difference.
Many of the Soft Proof ICCs we have supplied over the years are based upon various curves structures and setups, etc. and we simply make them available from time to time.
You really want to make your own following the Create ICC instructions on the QTR site if you need precision. And then you need a well calibrated display to make them useful.