I'd agree with Stephen, I'm not sure I see the point of holding at 620. I personally prefer doing a bubble squeeze by slowing down, not holding, at about that point. If I need a bubble squeeze I go down to about 10C dph between 645 and 670 or thereabouts, then add a hold at 670C to finish.
I might question how much air will actually be pushed out during a bubble squeeze if the pieces are stacked vertically inside the mold--the air needs to move past the glass and that's harder if several pieces above are softening and coming down at the same time, even with fiber paper.
I typically cast at about 812C, but my holds are much longer, usually at least 3-4 hours. That gives the glass time to become viscous and slip into mold crevices without the excessive flowing and scumming you can sometimes get from higher temps.
I would anneal for the entire thickness, i.e., from the outside edge to the outside edge, myself.
Only thing I'd really worry about is the thinfire.The glass will flow down strongly (which is what you want), but it tends to pull the softened refractory with it. It's not too bad with the fibre paper, but burned-out thinfire is 'way too delicate and can fold into the flowing glass and become trapped.