It is not possible to be precise due to the slight variations in sail size, the position of the bow fitting relative to the stem and the height of the pole attachment to the mast. (The latter is normally about gooseneck height).

However here are guide lengths for three popular types of foresail:

 When going down wind it is well worth using the pole because he goosewing effect gives more power and improved stability.  On a broad reach set the windward sheet to be angled well forward.  When using the pole for the first time check that the sheets are just long enough to allow this.