One additional detail why the Wabet room is the New Year room and with could be said to be the Sirius room:

That symbol of the sun rays shining on the Hathor head is according to some interpretations the symbol of light shining on the first day of the year onto Hathor.
And the Hathor head in the square itself is a symbol for the Hathor temple itself.

If they had some observation mirror raised through the ceiling window or were standing on the temple roof to check about the first appearance of Sirius, I don't know.