Welcome to the forums!!

I follow a pretty simple rule for this. It does depend on the question a tad, but essentially, the stimulus should be in there enough that
if you know the stimulus, it is easy to tell you are using it.
If you don't know the stimulus, we shouldn't be able to tell what it is.
For example, if you write a story and keep referencing an object without any real reason, I can safely guess that is your stimulus. If, however, you reference the object a fair bit but not a lot, and it makes sense why you are doing it, I can't know for sure! It could just be your story. That's the balance you need to strike.
So you need to incorporate it a fair bit, enough to be clear for your marker but not so much that is it clear to a general reader. The more skilfully and naturally integrated the stimulus is into your writing, the better your marks (for that aspect) will be
