I didn't set the thresholds with any particular energy capacity in mind. Instead, I set the "critical" threshold at 10.5V, since I read at http://batteryfaq.org/ that discharging a lead-acid battery below that level is a no-no. Then, I just looked at a voltage-vs-time plot of my light system discharging, and eyeballed the battery voltage that corresponds to "about half an hour until 10.5V" with typical use. This came out near 11.5V, so that's where I set the "low battery" warning threshold.