Let me try one more time... let's say you are mixing up 100ml of juice in a 100ml cyllinder. Your recipe calls for 1% of a particular flavor. Let's say you squirt 1ml of flavoring into the cylinder, and half of it sticks to the wall.
If a half ml sticks to the wall when you wash out the cylinder then you only mixed 0.5%, not the 1% you think you mixed. The same could happen with nic, where your juice ends up with a lower concentration than expected.
As a general rule, when you are mixing tiny concentrations of various fluids in an interim mixing container, you want everything reasonably well mixed when you pour it into your final container, or at least not stuck to the container walls. Unless you can cap your cylinder and vigorously shake it, I don't think you can do that.
Edit: I know from observation that up to 2-4ml of eliquid remains in a 100ml cylinder after the initial pour. The exact amount depending on my patience and if I pre-warmed the VG (I only use a cylinder to measure large amounts of VG)
Easy peasy, don't put your flavoring into the graduated cylinder. When I am making a batch, I measure the flavoring first and transfer it from the syringe into whatever bottle I will be shaking it up in. The cylinder is used to measure my pre made nic base, then that goes on top of the flavor in the bottle. Before pouring the nic base I even fill and shake the syringe several times with the nic base to get as much of the residual flavoring into the batch.
I use the same process when mixing nic base. Measure the concentrated nic. Next measure the PG, then the VG. Each time I pour I leave the cylinder propped and resting on the bottle at an angle and give it a few minutes to fully drain. Polypropylene cylinders drain better than glass, and if you want to be really ...., always final rinse with distilled water to prevent mineral buildup on your cylinders.