Hi Carel,
With the leading herds starting to cross into the Mara, headed north over the Sand River, my guess would be that we see the vanguard swing north and west into the Triangle to cross the Mara River in the next ten days or so.
The river itself is still very high, the heavy current in play following late rain will prove a pretty strong deterrent. Open fields of nutritious red oat grass south of the river will hold the western most herds in their journey to the river. Large numbers remain strung out, spread from the central Serengeti around Seronera, reaching as far north as Sand River and the Mara itself.
Keep well,
Alex