Considering you start with either 30 or 42, depending on rune-page, you have 130-142% EHP vs magic damage to start. Adding
grants an additional 40% to that, which is a 21-31% increase in EHP. This means 190-280 extra health vs her using your 900 HP example.
Toss in
, which is almost non-negotiable for laning, you're adding another 303-328 EHP. In total,
(720g)+
(150g) grants you an additional 493-608 EHP vs her that you wouldn't have had every time you back. That's not including HP regen, which is more significant than you think throughout the course of the 8+ minutes of early laning.
So for the low investment of 870 gold early on, you invalidate 493-608+ damage she would have dealt, which gives you an approximate value of 1314-1621 gold in health, not to mention the gold you denied by not feeding. That's not including the HP you have from
and the EHP reduction from her runes/masteries, which makes that early negatron far, far better than what I'm even suggesting.
If you really think she's going to be an issue, you can go from that
directly into
. Congrats, she basically can't fight you now. Finish that Abyssal, it becomes worse for her, and you're setting yourself up for an extremely good mid-game, while having the zoning threat vs Annie to be sure she cannot engage safely.
This means she can never all-in you at basically any point unless you're simply being stupid.