If you're planning to keep the height at 2.5, then the 1/3 rule would have you cut when the grass is 3.75. The new grass may not have germinated and you don't want to be sucking up seeds with the mower. I would wait a while for germination to take place and mow at a high setting. Then in a couple of days, mow lower, keeping the 1/3 rule in mind.. Mowing the new grass will encourage tillering. You do not want the new grass to get long before cutting. I cut at 1 inch just before overseeding. In 10 days the old grass was around 5 inches and I cut it to 3. Then I cut it to 2 and have kept it at 2. At this point I am cutting both old and new grass. Sometime in October I will go up to 3. The shorter height now is letting the new grass have plenty of light. Do not water on the day you cut until after mowing. Be careful with turns because the ground is soft. Try to make your turns on hard surfaces or adjacent areas.

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