Feeding question

Jay • Mama to a little boy🐻

So my 5 week old is eating about 4oz of formula every 3 hours during the day and going about 4-5 hours at night. He sometimes wants 5 oz but I never know which feedings he’s gonna want extra. Should I be making 5 oz bottles every time or just make another oz if he’s still hungry after the 4?? I don’t wanna waste formula but I also feel like he’s on the verge of needing that extra oz