Go with a rating that will cover you 75% of the time, and supplement with a pad if it gets colder.
If I could only have one underquilt, I would probably go with a 10°F uq. But I sleep very cold. Normal sleepers would probably be fine with a 20°F uq. I have used my 20° and 10° underquilts even in the hottest nights in summer. For venting, I just loosen the cords on the ends a bit, and relax the secondary suspension. Works perfectly for me.
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