First day away from BF how much milk

[emiwo]

My DS is 15weeks old and im about to escape for a whole day for the first time. I havent left him for no longer then 3hours before and hes only ever taken 3oz of the milk iv expressed when iv been away. Hes purely breastfed
He weighs 16lbs 5oz/65cm (big lad) and i dont know how much milk hell take in a daytime he feeds every 4hours. Plus i wont be nearby so i really need to make sure iv expressed enough. I can only ever express 2oz at a time so i dont want to waste any its breaks my heart ouring 7oz of defrosted un used milk down the sink,

Any advicewould be great thanks

I left DS2 for whole day (9-5) at 12 weeks (weighed much less than 16lbs), he took 2 feeds of around 4oz each.

I was also worried about wastage so for each feed left 2 bottles, told my mum to warm 4oz first and not to warm second bottle with extra 2oz until first finished so I knew that a max of 2oz would be wasted.