Formula storage question!

I've recently started supplementing to adding a powder formula as my supply has been dipping. It's a real pain at night if I run out of BM to make a bottle. It says "once feeding has begun" use within an hour. Does this mean I premake it before bed and keep it next to me until he's ready (even if it's a few hours before he actually has it) or does it generally mean once it's mixed used within an hour? Would be so helpful to find a solution to not having to get out of bed, I am trying to wean him off night feeds but we aren't there yet and he pretty much feeds and goes straight back to sleep so I'm okay with it. (We co-sleep)