What is K-factor?
The fetched Omni K-factor page describes K-factor as the position of the neutral axis within a bent sheet, expressed as a fraction of the material thickness.
K-factor formula
K = (180 x BA) / (pi x theta x T) - Ri / T
t = K x T
Here BA is bend allowance, theta is bend angle, T is thickness, and Ri is the inner bend radius. The neutral axis distance from the inner face is then K times the sheet thickness.
Source example
Omni's worked example uses a 1 mm sheet, 1 mm inner radius, 90-degree bend, and 2.1 mm bend allowance. That gives a K-factor of about 0.337, which is the default shown here.