Power Attack is one of those feats which I've read is a statistical must-have for melee builds but which I'm always hesitant to make use of since the penalty to hit makes me nervous. How do you properly gauge when to use it? People will say it depends largely on the AC of your target but, for a combat that lasts only 3-4 rounds, spending a round or two trying to determine the exact AC and then running some numbers to figure out whether you've reached a viable Power Attacking opportunity seems too time consuming to be worthwhile. Can someone to whom Power Attacking is second nature help me figure out how I should go about getting the most out of this feat?
For reference, I'm playing a 5th level Synthesist who is large, with 20 Str, and with four claw attacks; dealing approximately 1d8+5 per hit.