If you look at this way… You stated you used a total of 8 quarts of new fluid. If you start with a full system at 9.5 quarts of old fluid, you then drain and replace 4 quarts with new fluid. You’ll have 5.5 quarts of old left over in the system. Once you run the trans, 4 qts of new and 5.5 qts of old gets thoroughly mixed together.If you do double drain like what I did you'll get more than 90% of the old fluid out. Tranny flush is a tedious and expensive maintenance if you don't have the special tool.
Then, you drain another 4 quarts of fluid. But this time it is mixed old and new. Assuming it gets evenly distributed and mixed. You’ll approximately remove 2 quarts each of old and new fluid. So you end up with 3.5 quarts of old fluid left over and 6 quarts of new fluid in the system. That is 37% old and 63% new with only using 8 quarts of new fluid.
Continuing with the drain and refill with only 4 qts replaced. You be at 1.5 qts old and 8 qts new. At 12 quarts of new fluid cycled. Your looking at 16% old and 84% new.
To finish up, you again drain and replace another 4 qts. That would be a total of 16 quarts of new fluid used.
If you don't believe me, You can test this with 9.5 quarts of water with food coloring and see how long it takes to totally remove the food coloring.