Overfeeding or underfeeding? What's a new Momma to do?

My 5 month old son is an AWESOME sleeper. He used to wake up around 5 am and go to bed between 9 and 10. He took about 6 feedings of 5 oz each per day. But in the last month or so, he has been going to sleep around 8 and sleeping until between 7-8 (sometimes 9 if we put him to bed after 8). So, his new schedule now gives us 12 hours of wake time. 8 oz a feeding times 4 is 32 oz but 8 oz a feeding seems like it might be a little much for him (he sometimes gets a little pukey). I don't want to start waking him up to feed him and ruin his good sleep and he doesn't seem to be crying for more food if we cut back to 6 or 7 oz but then I worry he isn't getting enough. Any tips/advice?