When to start/stop the timer

Hello,

I don't understand if I should start the extractuon timer when I start the pre infusion (ie pushing the button) or when it start pouring coffee

I also don't know if I should stop the timer anc extraction when I reach the desired weight then instantly remove the cup

Or stop a bit before so that it finish dripping to reach the weight, and if I do that I don't know if I should continue the timer or not..

Thanks in advance