Well Justin, as you know So Cal is pretty dry in the summer. At the time, I was out of practice and hadn't bp for a few years so my estimates were awful. I guesstimated that on a day hike I drink 60 oz so 200 oz would be plenty for 2 full days. it was a June heat wave, I camped on the exposed summit, I got an altitude headache, by evening time I drank 150 oz. Didn't sleep good, thirsty from the dry air, and thinking my headache was due to dehydration, I drank the remaining 50 oz during the night. Woke up the next morning with no water, and 18 miles to go back to the car... with a pounding altitude headache.
when I found a creek at mile-15 (3 miles from the car) I saw a kid pissing in the water. I was tired, annoyed and just kept going to the car. had water there.
so lessons learned, now I bring extra extra water on dry terrain, and I cache many small bottles along the way. It's super heavy at the start, I lighten up as I drop off the cache, but on the way back it's very motivating to keep moving to the next mile/cache for a mini reward.