My husband gets therapeutic phlebotomies and his doctor writes the orders to remove a specific amount. In his case, they remove 250 ml every other month. They actually measure the output on scale to make sure they don't take too much. Your lab should not be assessing the amount visually. They need to be more precise. The standard therapeutic phlebotomy usually calls for the removal of 450 -500 ml of whole blood.

They can also do a process called apheresis where they pull out only one element of you blood and replace the rest. So they could take more red cells than whole blood donation because they put back the plasma and platelets after they spin off the red cells.