Piqued my interest enough to check. Ended up being part misremembering, part "your Galka so fat..." joke by SE.
For Elvaan the breaking point seems to be right smack in between 6.4. I was able to get to the very edge of 6.4 >> 6.5 and lose it, and get to 6.4 >> 6.3 and gain it. Stood at each point for 5ish seconds to ensure the affect had time to take effect.
Willing to bet the 6.1 I was thinking of came from testing with a Taru. This'll be some good information to be aware of if you've got either extreme of races in your party, knowing that you've got a quarter of a yalm more for Galkas compared to Elvaans is kinda cool. Checked using spells of the same day and weather to insure there wasn't anything like that in play as well and didn't see anything.
And yeah, cape aug is +%. It's kinda silly to put such a powerful effect on that cape IMO, any future Geos without it are gimping their Entrust by nearly a full minute. Although it does make me want more +duration gear so we can see Entrust approach something more like 80% upkeep. The whole no bonuses deal is enough of a negative effect for it IMO.