At the point midway between them, I'm seeing RSSI of about -70dBm or -72dBm from each AP, depending on which device I measure with. So that's right about where iOS would think about switching anyway. I've been considering adding a third AP in the middle of the apartment, but I now see it would accomplish very little unless I can get the phone to switch sooner than the midpoint --- hence my interest in 802.11k/v. Meanwhile it seems like a lot of the problem is slow reassociation; I can see that that's order of a second or so. These Zyxel APs claim to have 802.11r, but they're very clearly not actually doing it, so I suppose that's another thing that's not enabled in standalone mode.I assume you have your APs close enough so the signal does not drop and not too close to where they will not roam.