How much formula how often?

I just switched from breastfeeding and we are still trying to find a good amount for my 13-14 ish pound five month old. Started with 7 oz every 3 hours like he did with breast milk but he would spit up the formula like crazy. Went down to 5.5 every 3 hours which seemed to work, but now he's sleeping through the night (used to do one feeding in the middle of the night) so now I'm worried he's not getting enough because he's missing that feeding. He has to be on Alimentum for milk protein allergy. Thanks for your info!