I mix my Sherman OD with Tamiya paints by the bottle. 1:1:1 Flat brown, flat yellow, flat black. All go into a large testors gloss coat square bottle.
When it gets half way low, I add one more bottle of each.

I used to use a baby Tylenol syringe to mix. 2ml at a time or less in graduated marks. But once I got my color decided, I just go whole bottle.