? on re-seeding part of fescue lawn

I reseeded part of my fescue lawn and it germinated well. I'm curious how much water new lawn needs vs old lawn. I'm also curious if I am better off trimming it with my electronic weed cutter instead of my gas lawn mower. In the past it seems like I've killed new lawn even after waiting six weeks before mowing. Also, at what point in time does a lawn benefit from starter seed fertilizer versus regular fertilizer? I wouldn't mind putting some on the lawn to get the roots strong quicker.

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