Bobbin Winding

What’s on my loom right now (November 2013)? yards of Warp, at 20 threads per inch, for 39 nine inches wide in the reed; Is 780 warp threads. How long will it be? I hope for 4 yards of usable cloth, but one can never be exactly sure that one will not run out of weft thread an inch or two short of ones goals.