If she couldn't do 200k with We Found Love and could only do 205K with Loud (which had 2 #1 singles before release), she won't do 275K with just Diamonds Let's use common sense. She'll do 150-170K. 180K max.
Good luck to her.
We Found Love is a generic dance song which isn't a typical album seller, Diamonds is much more of a album seller. I think these predictions aren't that far off tbh.