You need to understand the Strength Factor system to understand the attendance concept.
If 50% of the people in the top 100 show up, and an equal number of people not in the top 100, what will the SF be? Assuming half of the people in each bracket (4 top8, 4 top16, 8 top32, etc.), it works out to 2.02, half the SF of 100% attendance event.
If P% of the people in the top 100 show up, and an equal number of people not in the top 100, what will the SF be? It works out to SF=4.04*P. (For P, 100%=1.0, 50%=0.5, 0%=0.0) Since this is an approximation, I use SF=4.0*P, or equivalently P=SF/4.0.
This concept of attendance is roughly equivalent to that of the SF. A tournament with a SF of 1.0 will have about 25% of the people in the top 100, give or take a few. A tournament with a SF of 3.0 will have about 75%. And so on.
Now you need to know how to predict the SF for tournaments, and how that will affect your result there.