it depends on which promix you want because technically, each kind of mix has a different percentage. it's listed on the msds sheet for each kind but they are around 15-20% and sometimes people like to add more so it really depends on your personal preference. so if 3.8 cuft of compressed peat expands to around 7 cuft and you have 2 bags that 's 14 cuft so that's 2.8 cuft of perlite for both bags of peat.
thats 20% so if you wanted to round up to 3 cuft that would work well too and makes for easier measuring. 1 cuft=7.5 gallons if you need that too. much cheaper to make your own promix.