Whatever makes sense to you..
i only count time after it accrues. That's what makes sense to me. It's more about familiarity and consistency. N or N-1, is irrelevant as long as you know which it is.
nutes won't fit into that increment anway unless you only water 1/week? You might do something different at start of week and need a correction later in the week, lol. So, don't worry too much about it. If you need a precise log of your fertiliation over time, you'll need to do it on your own as the format here won't do it well. A spreadsheet with date, dosage, volume of water etc would be what you need. Otherwise, your base procedure/dosage for that timeframe is plenty good enough for a GD diary.
or, you may not flip to flower exactly at the start of a week... So, i don't categorize a week as flower phase until the majority of that week is on a 12/12 schedule, but the 'week' is always accurate to total days above ground. Week 10 is always day 70 above ground for my diaries. (some exceptions due to my availability to take pictures or just being lazy, of course). I always note when i flip and give 2 different counts in the weekly comment - "Total days above ground" and "total flower nights"
Germination week is just until i get most of the sprouts above ground, then week one of vege is 7days after some middle ground for that range of sprout times. That's also a bit of a grey area you can't fully account for given GD's format, unless you make a diary per seed.
As long as you know what you do, it's "correct." It's a tool for you to use. Don't worry about anything else. Consistency is the key for anyone looking at your diary, too.