How about instead of plucking fantasy figures from thin air and then winging when they aren't reached we use our brains to predict a crowd?
Souths v Easts have only drawn 30k+ to two SA H/A games. One was the exceptional 59708 to round 26 in 2013. Today will be less than that as there is much less at stake.
The other was 32125 back in 2007. Again, late season in Round 25 so more at stake. Souths have come on a bit since then , so this is a reasonable high estimate.
Traditional R1 fixtures at the SFS draw from 27k to 35k - but the sfs is a more favourable venue for both fansets, and now is not the first round, so 35k is probably over.
Final estimate - 30k to 32k, so, I don't know, 31679
Calls for 40k+ are but wishful thinking and saying "this game should really bring in an x crowd" is a baseless assumption.