Ray, with regards to "hands up", I set the car at whatever height is comfortable. Even working on the Spyder engine is easier, being able to raise the car so the engine is at waist/elbow level, with the clamshell open. I agree with you about working under the dash, any height off the ground helps.
My garage is 19 x 19 feet, so size "small", with a 7 foot ceiling. I have put two cars in there(Audi allroad and the Spyder) and a snowblower, but it was TIGHT. Now I keep the Spyder in my enclosed trailer and the Cayman in the garage or vice-versa and there is plenty of elbow room.
The Quickjack seems about perfect for what you do with it. I just need more height, probably because the Spyder is smaller and lower and my knees are worse. Plus my concrete isn't as smooth as yours, mine is 70 years old and rough in spots. Sometimes my mechanic's seat and creeper get stuck in the rough.