anything over 7.5% x AGI is deductible

Let's say your adjusted gross income is $100K, 7.5% of 100K is 7500. 

If you pay your obamacare premium, anything over $7500 is deductible as medical expense in itemized deduction.  

But sometimes you might not eligible for itemized deduction.  Let's say your other deduction is only $20K,  and the standard deduction for married couple is $27750, you are 7750 short from itemized deduction, so your health insurance premium has to be more than 7750+7500 = $15250 in order for you to start deduction.