Just because something has a calcium atom in the formula doesn't mean you're taking a calcium supplement. You're not, so no worries there.
140/90 is more than just time to address your BP. That's stage 2 hypertension and should be treated with two meds according to
current hypertension guidelines. Never mind calcium and all that, hypertension is your biggest enemy when it comes to afib because there's a direct causative link between the two:
Hypertension --> atrial stretch --> fibrosis --> afib
If I were you I wouldn't be cutting the losartan in half. In fact, I'll be surprised if 50 mg of losartan brings it down far enough all by itself. If you went to my EP, he would insist on getting your BP down at least under 130/80, and preferably under 120/80.