Summer climate isn't appreciably different there than in Michigan. I always use a 25/30F bag (-5/0C bag, I guess) here. Remember, bags ratings are what keep a generic adult man in long clothing OK, not necessarily every person at a comfortable temperature. Worst case, if it is too warm, I just unzip it. Most recommend going 15-20F (8 to 12C) lower than the lowest temp you expect to encounter.

https://www.rei.com/learn/expert-adv...eping-bag.html