Rotameter Mixing

[GM42489]

I am using manual rotameters to mix concentrate and water in-line. do I need to be concerned if the pressures of each stream are different? How can i be sure that backflow does not occur if my water line is higher pressure than my concentrate. 

what kind of range of mixing can I achieve? Can i mix 0.1mL concentrate with 100mL water if the pressures are the same? how much will variation in house water pressure effect the ultimate accuracy?

Thanks