Thanks, aa440. Reading that thread and others, there doesn't appear to be any 100% confidence rule. It's possible to get the reduced credit, but others report getting full credit (including EQM).

Me, I booked a UA fare for YYZ-MIA-YYZ, on AC metal. Price was near-enough identical to a Tango fare, so I chose the UA ticket on the chance that I might get full miles -- and I did, including my 50% E bonus.

Unfortunately our corporate travel policies generally do not support the selection of fully flexible fares, so if Tango is available, Tango is what I need to take.