Are you looking for activity or close to within a certain amount of points to terminal gravity?
Yes, mostly activity slowing down. Timing is not
that critical, but definitely toward the end when things have slowed down. I guess with 5-15 points left, I sometimes measure gravity when I want to make sure.
There's no concern with some of it being left in the fermenter? How are you incorporating this so it doesn't fall to the bottom? I assume you're giving it a mix or stir without concern of oxidizing because fermentation is still active?
The syrup is thinnish or thinned down, it pours well when added. It's not thick, honey-like. It should disperse and dissolve quickly. It gets added through a 1" port the airlock hole in the lid, while CO2 is streaming in through the airlock hole.
When in doubt or to help faster integration, I stir it through the same 1" port in the bucket lid with CO2 streaming in, same procedure I use for dry hopping.
Any air (oxygen) ingress
after fermentation has started should be avoided.
Oxygen added when pitching yeast gets used up by her to propagate. Whatever remains gets driven off quickly by fermentation CO2. It doesn't linger around for long.