"do the calculations with generous margins of error, and be guided by the result."
With soft strips, the data to do calculations is guesswork.
Most people get their license on hard, relatively long, runways. I was asked to demonstrate soft field technique during a C172 checkout for rental in the US, on a long, hard, runway. No problem, but very different from the real thing - no feedback.
I learned the hard way that some runways need methods different from anything In the standard manual and textbook.