In a couple of week I am doing light and sound for a band that needs to balance acoustic instruments against acoustic drums in a church. I have yet to receive firm information on the type of speakers over and above a photo, or amp power (in any format let alone RMS) so I anticipate taking my PA, but the installed speakers will be less intrusive. But the speakers are arranged into a row on each side of the church, and even if I had the speaker models and amp power, I have no idea how to calculate the SPL as compared to a typical arrangement. Granted that's likely a complex question, only fully answered by a full simulation of the space, but are there any rules of thumb? Otherwise I will assume no gain from additional speakers (no +3dB or +6dB as no coupling). Would that be reasonable? Against the drums I'd ideally like to have 110dB SPL (@1W) with 3dB of headroom available, even if I don't push over 105dB, and assuming typically inefficient installation speakers I expect that would need in the region of 250-500W RMS. Luckily (in this instance) lots of bass isn't required. In terms of PA, taking just a pair of speakers, I can have 500W into two 95dB SPL (@1W@1m) speakers on tap, so that should be plenty, but would be less discreet if required.