Mixed feeding older baby- quantity?

[squizita]

I bf my 8 month old but if I'm out and there's no expressed milk handy we give SMA ready made.
We do it purely by "demand" and she's quite happy to take it (eg last night I bfed at 6, at 8.30 she guzzled 5oz SMA!) but the fact that quantity for formula is by age and weight has me a bit confused as (1) she's 2nd percentile so not average weight for her age, do I go with weight or age and (2) as I bf usually, it's not for the whole 24 hr, so how do I calculate that?

Or should I just let her drink her fill as my supply is settled and she's not exactly at risk of excessive weight?
I buy the cartons of approx 5oz which she prefers to powder.

I'm back at work for a fortnight in July and will buy the bigger bottles of ready made then.
At night I bf twice so my supply is maintained I hope!

Mine is 8 months too. I've moved her totally on to formula over the last two months. Over the last 2 days I've written down what she drinks (mostly to stop us wasting formula), and even though she doesn't ever finish a bottle, she's still drinking 900 - 1000ml a day. On top of three fairly decent meals. She's 45th centile (up from 9th at 4 months). Considering mine doesn't finish bottles, I'm assuming she's getting exactly what she wants/needs, so if I were you I'd 'let' her do the same. If they've self regulated for their whole lives, they must be slightly alright at it.