Ok, so other projects aside, I got around to another test. Took my 9 vids, shared out of FCP via compressor, MPEG-2 with AC3, 5-7.5Mbps vbr 2 pass best encoding. I can see right off the bat, not all 9 will fit, so I pull the first 8 into Toast - it says, 23.20 on 1 disk, 110 M remaining. I set re-econde never and hit the big red button. A few minutes later I get "There's not enough free space on this disk. 24.64 GB are needed, 21.31 are available. That is 1.44GB of bloat/miscalculation in the multiplex process - can't be right.
Just for grins, I pull video number 8 off - it is nearly 52 minutes. It says, 21.93 GB on disk, 1.38 GB remainaing, so of course when I hit the big red button, it multiplexes and tells me that 23.32 GB are needed and 23.31 GB are available.
So, yes, I can re-encode to get smaller videos or put fewer ones on, but nothing explains why Toast is doing such a terrible job of calculating the space required - or why it needs to bloat the disk with 5% overhead.
I'm still waiting for customer service to provide more than the stock - here's how you encode response.