For a similar situation, but this time David has from time to time tasks which will not take him 100% - sometimes 20%, sometimes 50%.
But he also has tasks which don't have limitation of Units, and can be advanced any time that he is available.

Is there any method to overcome Project's inability to automatically lower Units while prolonging the tasks length?
(This time there is no constant Unit value that can be used as it was with Giovanni's 75%.)

Thanks!