For The Rangers match, I would give them an initial allocation of 1,500. If Hibs supporters snap up the remaining tickets in the East and West, then the South can be opened. This already happens for every match not involving the OF or Hearts. The precise split can be determined by how fast the South Blocks sell out. Just like the Aberdeen match.
If the tickets are slow to sell then the remainder of the South Stand can go to Ibrox. Very unlikely I think.