With the front foot buckled in, and the forward lean established, gradually move the free foot rearwards. The goal is to find a width that allows the rider to stand equally weighted, hips facing the direction of the front foot, without muscle tension.
If you go too wide, the tendency will be to turn the hips to avoid muscle tension. The resultant stance width may seem to be narrow, but again, you are looking for a functional baseline. If the desired stance width is not possible due to a lack of inserts, find another board.