The sweet spot is simply the location where it sounds best. You won't know where that is until you go to that place and listen. Could be the front row, could be 5 rows back to the side, could be row 12 center... expecting it to always be in a similar location, venue to venue and act to act, closes the door to the possibility of making some outstanding recordings in what may be to you an atypical situation. If it's atypical or not simply depends on what your usual recording experience is like. For me, the last 3 things I recorded sounded far better in the front row than anywhere else. Two of those were amplified, one used the FOH system, one was fully acoustic. One was in a small but high ceiling shoe-box shaped gallery, one was a medium sized 1940's deco theater and one was a smallish club. None of them had stage fills, but as Mr. Willie notes, those things may well be the feather in your cap.
Keep your ears open and aim to put yourself in the empirical sweetspot instead of the theoretical one, even if its the next time you show up, dependent on all the other important variables like girls, wives, vocals, pianos, fill speakers, soloists and drums.. drums are especially important, pianos are hard, girls can go either way.