Measure the height of the cars in question (to the highest point on the car). Most sports cars are in the 45" - 55" range. The popular stacker lifts have a top lock deck height of 72". Add the 2 up and you get your number. Also remember that you usually go slightly above the locking point then lower the car onto the locks. That takes a couple more inches headroom than the absolute measurements.
I'm building a new garage and am going through the same calculations. I think the minimum height to be practical is 10'. You certainly can go lower if the only cars you would EVER want to put on or under the lift are small but it will **** if you decide to get into street rods or restorations and find you need more headroom.
You also need to consider the garage door intrusion into the garage and everything that's hanging from the ceiling. Your effective maximum height is to the lowest point of "stuff" hanging from the ceiling in the area you are lifting. For the garage door you can convert to a high lift track that will get the door up to about 12" below the ceiling. For each inch you move the track up you can subtract that much from the rear intrusion of the door. Also remember that if your garage is deep enough you can back the car onto the lift and allow the hood of the car to rest under the garage door leaving the top of the car open to the ceiling.
Example, I'm building a new garage and started with a 10' ceiling. I originally was going to do an 18' x 8' door/opening until I calculated the rearward intrusion of the door. My final plans look like I'll go to a 11'4" ceiling height (block construction so the course height goes up in 8" increments, cost to go up is $0.70 psf per 8" course). I'll also drop to an 18' x 7' door. Using a high lift track the door will end up ~10' 2" above grade and only protrude 4' rearward. If I use an I-Drive opener I have nothing but 2 4' tracks 12 - 16" below the ceiling as obstructions. The other alternative is to use an insulated slat roll up door but I haven't got pricing on that yet. If I go roll up I'll do an 18' x 8' opening.
