How does maximum points capping work?

[turtlemichael]

My understanding of my Platinum Card is that once I earn 300,000 points in a year I will only earn points at either 1 or 0.5 points per dollar spent - no more 2 and 3 points per dollar. In the last few days I seem to have reverted to 1 point per dollar on restaurant etc transactions. My Amex year starts in January. Since January I have a sizeable margin less than 300,00 points. In the last 12 months (back to October 2015) I am also below the threshold. My reading of the T and C's is that it relates to my Amex year so in my case from January but the wording is not totally clear.

Does anybody know how the time calculation works before I contact Amex for an explanation?