I think you mean a John McMillan not Myles McMillan got all 600 acres.
One John McMillan was of the 24.5 Lochiel, Another was John McMillan of 22.4 Lochiel.
I can not remember whom the 3rd John McMillan was. The person writing down the land grants really did goof. There are a few McMillan whom have researched the 8th and 9th concession McMillans. I have only where it intermixes with my family.
Anne Neuman