I'm similar to Natnee.
I'm combination feeding on demand, (I start each feed breastfeeding and then top her up with formula). Sometimes she'll take 1oz sometimes she'll take 3oz. Preparing that and trying to breastfeed when she shows her feeding cues every time would be a complete nightmare.
I guess I could BF then stop when I normally burp her and make up the formula, but she'd probably fall asleep on me and not take enough calories to put on weight!
What I do just now is add her colic medicine to 3 steralised bottles, add almost boiling water (in order to kill the bacteria in the powder), add the formula powder, and immediately put them into the main body of the fridge. I do this mid morning to last us through the day, and then after dinner to last us through the evening [and depending if she's drunk those I'll make more for the nightshift (but 6 normally lasts her 24 hours)] I then heat them in the microwave for 20 seconds when needed (yeah, I know, another no-no! I'm very careful making sure they are stirred really well to ensure there are no hot-spots)
I work in Clinical Governance in the NHS and made the informed decision to make the formula feeds in advance and warm them in the microwave. I understand that if I get it wrong she might get an upset tummy, but the benefits outweighed the risks for us.
I can see how we'd be able to plan well enough in advance if she was soley FF on a schedule though so maybe we'd do that if it got to the point where I had no breast milk left...