Thats a rather boring idea, I'd rather see the flack jacket degrade with every hit you take, and slowly your max health declines from 100 to 50 as the jacked gets weaker and weaker.

So perhaps each hit you take deducts .05% percent from it meaning you can take 2000 bullets before your jacket will reduce to nothing. And so the boost your flack jacket will give to your max hp is:
100-deductedpoints/100*50 rounded to a whole number of course