From my understanding you want to spray after preflowers have shown, select your branch or branches that can be treated independently from the rest if your plant and treat the branches at the nodes where buds will be starting to develop. It takes a month for them to flip and start releasing pollen even if you start spraying before bud sites are all defined.
I've done sts twice now and I got it right the second time when I made a stronger solution. Sadly the mother that chucked larfed out in the end and screwed me. I'm giving up on many if my seed creations and working new lines and the few more promising crosses I've ended up with.