For timing my clones, etc, I add two weeks to the time listed by the breeder.
For nutrients, I still don't know!
Most nutrient companies feeding schedules match most breeder flowering times, 8 or 9 weeks. This means on an indica, you wouldn't move to a bloom nutrient until it's already been on 12/12 light for two weeks, but that doesn't seem right because a lot of people claim that nutrient changes help the plant transition into flower. But if you go into bloom nutes right at the flip, you'll need to use the "extra weeks" option on the feed chart for every single strain. And that makes it look like you're starting too early.
It's mostly not an issue for me anymore. My flower nutrients use the same 3-1-2 ratio as my veg plants, just different stuff, so the timing of the switch doesn't really matter. I do a PK bump for one week, four weeks before flowering, like on the Canna Coco schedule. If I'm growing a new strain I just add two weeks to the breeder's longest time and that's usually accurate enough.