Change in sleeping and feeding pattern

Dear Marcus is now sleeping straight through from 11pm to 7 or 8am next morning without waking up in the middle of the night. I feel so blessed! He started this new habit since he was two and a half months old. But, now he only feeds about 5 times a day. I read in a Gina Ford book (Contented Baby Book), the minimum calories the child has to drink is 21oz a day. But, he's drinking less now. Sometimes, the total feeding could reach this minimum level. However, most days he just drinks up to 18oz a day. He seems pretty happy with it. I am currently giving him breastmilk and formula milk. I noticed that when I give him formula milk for the last feeding at 10.30pm, he sleeps through till 8am sometimes 9am. But, if I give him breastmilk, he'll usually wake up earlier sometimes 6.30am. So, I've decided to give him formula milk for his 10.30pm feeding. If he does not sleep by 11.15pm, I will give him another 2oz of formula milk. With formula milk, I can afford to waste. I don't mind if he can't finish. But, with expressed milk I really made sure that he finishes as much as he could.