Hi Alb,
What I would suggest is for you to use process logic to create your order entities rather than a source. The Create process step can be used to create entities just like a source, and you can put in logic that looks up the percentages.
I would also suggest you use timers or perhaps a source to just create trigger entities that tells your model when to fire events (perform logic). For example, in your above case, perhaps have a source that creates an entity at 7pm, 7:30pm, and 8pm. These entities then trigger the process to be executed that decides the percentage of entities to be created at each time.