Most of the time your Baffles are around 8".. Thats a good height for most skimmers and Return pumps,, ect... Now if your trying to Make them as high as you can but still have enough room for what the tank will drain during a power outage.. Thats a lil different.. IMO I would just make them 8" and not try to cut it so close. There really is noway to know exactly how much water the tank will drain till its setup.. It depends alot on how the plumping is how low the return is in the tank ,,ect... With a 90 and a 29 gallon sump you could probably go a lil higher but 8" you give you plenty of room.. Besides your water will sit about .5-1" higher then your baffles so figure that too.
