Sometimes the most useful research isn't a single study, it's the industry stepping back and agreeing on a standard. This is the National Animal Health Monitoring System's consensus recommendations on calf- and herd-level passive immunity benchmarks for US dairy calves. For producers wanting to know what "good" actually looks like on a national scale, this is the reference point the rest of the industry measures against.
"Consensus recommendations on calf- and herd-level passive immunity in dairy calves in the United States" — J. Lombard, N. Urie, F. Garry, S. Godden, J. Quigley, T. Earleywine, S. McGuirk, D. Moore, M. Branan, M. Chamorro, G. Smith, C. Shivley, D. Catherman, D. Haines, A.J. Heinrichs, R. James, J. Maas, K. Sterner. J Dairy Sci. 2020;103(8):7611-7624.