The green glow gives you a baseline, it's dependent on your connection as people have suggested.
If you notice green glow is too late, then you have a slow connection and need to go before the green glow normally happens. If the green glow is too early, you have a fast connection and need to wait longer after the green glow.
The point for that suggestion was to give you a starting point, and you can learn your timing by going earlier or later if needed. For most people, the green glow is pretty close to the correct timing.
P.S. I'm beginning to question my 1.25s assertion as well. It could be 1.5s. The most BPs I've ever actually counted on a Conduit is 18, not including the two initial ones from Apogee. If ready time is 1.5s, that would put the theoretical max at 20. If it's really possible to hit 24 in 30 seconds, I would think the more skilled SMNs could get into the 20s pretty regularly.
what number of bloodpacts should I target?
I usually say that 12 is passable, 15 is good, more than 15 is great. Note, that's just with Conduit, not including the two from Apogee before you start Conduit.
Edit: The effect lag has on this timing really makes it more of an art than a science, because lag is never constant. Sometimes it feels like my animation started a bit late, and so I might hit the next BP a bit earlier than I normally would. Sometimes two come close together, possibly because the first of the two was lagged like in the prior situation, and when that happens I don't try to go even a little bit early on the next one. Once you're practiced, you should be able to hit 15+ consistently.