This is just my opinion.
First, it's usually not a titer but a single-point dilution.
What dilution? It's truly arbitrary. Some go 1:50, some 1:100, others 1:200. It depends on what the facility medical director will accept.
Your colleague should collaborate with their medical director to determine what they feel is "safe". This cut-off will not prevent reactions, only minimize the risk.
Then from there, it should be a simple procedure of a single-point dilution, with a cut of 1+, 2+... or less.