Question

I have been trying to do Minimum Inhibitory Concentration to see the effect of short chain fatty acids on microbial growth. I have 2 problems 1) what dilution concentration should I use for Butyric acid? 2) How Can I make sure I add the same amount of Bacteria in each tube? I am using optical density to calculate the bacterial growth of my overnight culture but I do not know how I can have same optical density in each tube.