In this topic, we worked out that the two distances where the same. But I don’t understand.
If I walk my model with entropic force into an area that I need more speed to enter, do I stop at exactly just in or just out of 5”?
This is answered by the rough terrain rules, as found under “Terrain Types.”
If you plan to enter rough terrain, and you lack Pathfinder or an equivalent rule, you subtract first.
Suppose your model is SPD 6 and there’s a patch of rough terrain 5" away. If you would full advance 6" and be 1" into the rough terrain, you subtract the rough terrain penalty before you move. You now only get to move 4".
(Yes, that is indeed how it works. Hold on; nobody reach for the torches and pitchforks yet. 
)
In such a case, it’s generally smarter to say “I am not entering the rough terrain” and just advance the 5" and get to the edge instead. 
That is the general case for most physical terrain you see on tables, where you have to be inside the perimeter of the terrain to be affected.
Your example is a little different than that example, though.
In this case, both the Floodwaters and Entropic Aura say “within 5 inches.”
From Measuring Distances:
If your model with Entropic Aura is exactly 5" away from the model with Floodwaters, you are within 5" of the model with Floodwaters, meaning you are affected by Floodwaters.
If you can’t pay the rough terrain penalty to get within 5" of that model, you’ll have to stop short. You might be 5.00000000000000001 inches away, but you are more than 5 inches away. Your Entropic Aura will not affect the model with Floodwaters.
Does that make sense?
2 Likes
I am getting the impression that you are resolving that 4.99999999999……9. Forever and 5.00000……1 are different. Which is complex.
But you’re not. The model can not advance within 5”. So must be greater than 5”.