"K" has been defined equal to from 10 to 12 for copper
A engineer on one of the other sites has measured the value of "K" in the real world after his installations have been put to use.
He found an even wider range of figures for "K" I believe he said he made a spread sheet based on about 11.
For me it is just easier to use a VD calculator program on my PC.
It has settings for the type of conduit, AC, DC, single phase, 3 phase, voltage etc.
You can figure for Voltage drop, minimum wire size or maximum length.