In sheet metal design, the K-factor is used to calculate how much sheet metal one needs to leave for the bend in order to achieve particular final dimensions, especially for between the straight sides next the bend. Use the known K-factor and the known inner bending radius to calculate the bending radius of the neutral line. Then use the neutral bending radius to calculate the arc length of the neutral line ("circumference of circle" multiplied by the "bend angle as fraction of 360deg"). The arc length of the neutral line is the length of the sheet metal you have to leave for the bend.