The Rubens Variation no 1 is a siccative, and therefore you must take into account that what you put in the paint will dry your paint by the end of your session. If I plan to use my pallet for a couple of days, then I will not add it to all the paint.

But if I plan to use all my paint in a day, then I will add a portion to all paint. You can also add more or less with each brush stroke. That is why I keep a little dollop on my pallet.