The recommendation for NACA5 with its given parameters is 3.5m minimum, 5-10m optimum, 20m maximum. You are right that the meter recommendation to achieve the same inductance would differ if the speaker cables have different parameters per meter. However, the values are often not published for other cables. Braided and twisted cabled are not suitable and can indeed even damage the amp. However, Witchhat has “the Naim specialists” written large on their home page and their cables are suitable and work very well by all accounts. You will be fine with 7-12, but if in doubt, ask Witchhat.
By the way, FAQ here: