Mcq renal phys destroyed nephrons?

I am unsure, but if 80% nephrons are destroyed only 20% are working, for which I will ume GFR =1/5. But there is compensation, thus GFR=2/5. Thus the real question remains: is 40%GFR enough to balance blood urea or not. If not, then it will be contiuously rising; if yes, it would be 5mmol/L.