Kayfun KFL+ Plastic vs. Steel center section

[RuDawg7890]

So I have a KFL+(second batch), authentic. I've been using the plastic tank section. But the steel center section has me curious...How do you know how much is enough when refilling the tank?
Is it just educated guess work?

 

[Towmotor]

yup. Or..... I use the same syringe to fill it every time, just make a note the first time of how much it takes.