SB & bitrate calculation

[Séamus]

So, this is gonna be my first SB title, Alien Disc1 with the theatrical and director's cut version
I'm using DIF4U 1.4.7.0 BatchCCE & Scenarist 2.7

PGC's look like this:

PGC_1 (program chain): [Title(TTN): 1] [01:51:50.23 / 25 fps] (Programs: 32) (Cells: 106) (uses VOB-IDs: 2,3,4,6,7,8,10,11,12,14,15,16,18,19,20,22,23,24,26,27,28,30,31,32,34,35,36,38,39,40,42,43,44,46,47,48,49,51,52,53,55,56,57,59,60,61,63,64,65,67,68,69,71,72,73,75)

PGC_2 (program chain): [Title(TTN): 2] [01:51:05.06 / 25 fps] (Programs: 40) (Cells: 111) (uses VOB-IDs: 2,3,5,6,7,9,10,11,13,14,15,17,18,19,21,22,23,25,26,27,29,30,31,33,34,35,37,38,39,41,42,43,45,46,47,48,50,51,52,54,55,56,58,59,60,62,63,64,66,67,68,70,71,72,74,75)

So my question is, if I select both PGC's and flag them as VOBId demux I will get a bitrate calculation for the 2 complete PGC's if I'm not mistaken.
So after reauthoring I will end up with a smaller disc because of the duplicate VobID's. So how can I calculate a bitrate that will give me a full sized DVD5 ?

Thx.