Bad at numbers

The reason I'm bad at math is that I mess up with, like, simple interpretation of numbers. I'm fine with concepts. I'm extremely bad at correctly putting those numbers into contexts. 

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So, I'm trying to make my own set of grayscale inks, because I'm broke and you can make gray ink by taking black ink and adding water. And I want it to be repeatable, so I'm keeping track of the proportions of ink to water I'm putting together. 

Eventually I got to the point where I decided to convert the quantities of ink from drops to ml, because there didn't wind up being all that much of a difference between, say, 8 drops and 16 drops. So, I figured powers of four, rounded to the nearest clean ml number. 

And I got the number of drops to a ml by counting drips out of the needle I was using for all the measurements. 

What I fucked up was:

The needle is marked out by half-milileters, and I counted the drops as if it were marked out by whole milileters. That is to say, every line on the needle represents .1 ml., and I thought it represented .2. 

I didn't realize my mistake until after I had mixed the ink that should have had 5 ml of ink (about 50-50 water to ink) using 10 ml., and almost all of my remaining black ink.

So, now I'm just gonna mix them all together and then adjust them haphazardly until I like the spread of tones, because fuck consistency and I'm not going to try to do the math to figure out just how much ink and water is in all these bottles between them.