My suggestion would be to run the same downforce on both cars but not the fastest. That should give an indication of the difference as well but one wouldn't have to wait for the speed to gradually build all the way until braking. If one max out say 2 seconds before braking one will have time to actually check the difference. And run with driver view in such case because then there will be a numerical value of the speed, not a graph where it is hard to tell the difference.do a time trial session at Monza with both cars and run low downforce.
My suggestion would be to run the same downforce on both cars but not the fastest. That should give an indication of the difference as well but one wouldn't have to wait for the speed to gradually build all the way until braking. If one max out say 2 seconds before braking one will have time to actually check the difference. And run with driver view in such case because then there will be a numerical value of the speed, not a graph where it is hard to tell the difference.