It is not compulsory to use the silicone tube if you don't find a need to use it. Less troublesome IMO.
If you do use it on the syringe, it is important to leave some air in the syringe as you draw 7ml of water. After drawing, invert the syringe and all the air in the syringe will go into the silicone tube as you expel it. Do not expel the liquid from the syringe. Once the air is out of the syringe, invert the syringe again and all the excess water will empty out from the silicon tube.