Lower than it hit at 100 yards.

Honestly, there is no way to accurately answer that question. The one and ONLY way to know for sure is to take the gun out with whatever load you're shooting and actually shoot it at 300 yards.

To give you a ballpark estimate based on ballistics tables we'd need much more information than you've provided.

What is the weight of the bullet, its Ballistic Coefficient (BC), the actual muzzle velocity of the rifle, and height of the sight or scope above the bore's centerline. To get even more accurate, we'd need the ambient conditions like temperature and altitude.

Mike