I've had three Roadsters over 12 years. Always put in 8 quarts, it's always within 10% of the top mark.

What you are doing wrong is using ramps. Just remove the underbelly pan (you can do that with a small ratchet laying next to the car, no need to jack the car up or use ramps).
Then, use a low-profile drain pan capable of no less than 10 quarts (mine holds over 3 gallons) and drain the oil by sliding the pan under the car from the front.

I don't see why people have so much trouble doing this. And no, I dont' remember where I got my pan, but it had to be Autozone, Advance or O'Reilly's. I just shopped around until I found one of more than two gallon capacity but only a few inches tall. I change oil in the garage where the floor is level, if you are out in the yard, try pulling to car up to an area where the ground slopes down. Pull right up to the edge where it slopes down, and you can get to the drain plug easier.

But again, I just do it in the garage.

I will confess, my last two Roadsters didnt have the underbelly pan. My first one did but that did not stop me from doing this without ramps. I HAVE ramps, but I don't use them for oil changes.