SSB backstay antenna length
While the tuner will handle most anything, you want to try to be sure that the length is not a half wavelength at any frequency you might want to operate at. At a half wavelength long, the tuner may have problems as the impedance and consequently the voltage at the output of the tuner will get very high, if the tuner even has enough range.
If the length becomes greater than about 5/8 of a wavelength at any frequency, then you will have more radiation at high angles, which is detrimental to long distance HF work. On the other hand, the lower the frequency, the longer it should be for best efficiency. I consider this aspect less of a concern for a compromise antenna.
So, it somewhat depends on what you want to optimize it for as any length will be a compromise, but almost any length will work. Take the length (in feet) you are considering (including the wire from the tuner to the backstay) and divide it into 492 and see if the result is within 10% or so of any of the frequencies (in MHz) that you plan to use. If so, I''d change the length a bit.