I think its a question of experience.
In my early acrylic times
I always had pretty too much colour on my palette which I then used for painting postcards.
In the end I had so many of them that they would last my life time!
Then I slowly got accomodated to using my paints.
And I am not getting nervous anymore if the amount of premixed colour is too small: I just mix it once more
which seems to be another effect of getting more experienced.
Just go on painting, I'm sure you rather soon will know how to charge your paint adequately !