Don't forget that there's more to it than just the resistance of the cable. We're dealing with high frequencies, and so inductive and capacitive effects and skin effect come into play. These will vary from line to line, so any calculation of line length based on attenuation can only be an approximation.
Some years ago, when connections were being upgraded from ADSL to ADSL2+, which doubled the tone range used by the connection, it was commonly noticed that modems reported an attenuation which was ~3 dB higher than before. With VDSL2 the tone range is much greater, and more variable, so the frequency effects on attenuation will be correspondingly higher.