As mentioned above, Lee's little "Perfect" measure is as close to consistant as a powder measure gets for common tubular rifle powders. But, if it's not cleaned and assembled correctly it will dribble ball powders every time.
Thin flake powders will bind any measure. Ball powders measure like water in anything. Nothing is or can be highly consistant with coarse tubular powders. Poorly mounted - shaky - powder measures will vary more than solidly mounted. Varying lever operation will always produce varying drops.
The biggest problem with any powder measure is the loose nut holding the handle.