a good rule of thumb is to try to be at the point of an equilateral triangle, where the stacks on either side make up the other two points.
so...
if (s) is your venue's speaker...and (t) is you, the taper...
(s) stage (s)
(t)
that's usually a good place to start. if there's an opening act and the room isn't very full, you could walk around and try to find the "sweet spot" and tape from where it sounds best in the venue.