I've seen spreads lower than 20% for a Thursday or Friday afternoon less than 14 days out, when the cheaper WGA fare buckets are sold out. Looking at MSP-MDW one-way on the afternoon 9/9 (so sub-14 day), BS is $205, AT is $190, and WGA varies from $151-$179.

For an A+ member, it's better to buy BS over WGA at the $179 price, and break-even (but more TQP) at the $151 price. That is to say, if you have reason to use RR2 points for WGA fares in the future.