I'm familiar with the Sensi 2-part nutes if you have questions about it I can try to answer them and if I can't I know who can.

As far as when to start feeding them - if the leaves get pale and turn yellow you waited too long. If the tips burn you started too soon/heavy. It varies from strain to strain how quickly they need nutes and how much you can give them to start.

I always start off super light, like less than quarter strength and build up slowly from there. Obviously you don't want to try to do it wrong, but just remember every mistake teaches you more about where the limits are.