Lactic Acid:
Typical Concentrations in Beer:
0.2-1.5 mg/L
Perception Threshold (in neutral solution):
0.04 mg/L
So, 0.2-1.5 ppm would be what is normal in beer anyway - perception would depend on the base beer. For comparison, I dosed Coors Light at a rate of 1.5 mg/L lactic acid in an off flavors presentation I did and about 50% of our club members picked it out correctly.