To actually calculate a "titer", a standard is needed. I guess if you are looking at titer that is currently measured in human patients, you can find the standard. (For mouse, I usually just look at OD differences for comparison. Nothing soficsticated.) Otherwise, I've pasted two links that hopefully can help.

https://www.researchgate.net/post/How_should_I_calculate_antibody_titer_from_ELISA_result

Article Determination of the Specific Antibody Titer

The ELISA assay follows an inhibitor response where one can determine EC50 of the drug (inhibitor), as one increases the dilution the response (OD) goes down.

The EC50 of a drug can be obtained by 4parameter logistic curve fitting model.

Some plate readers come with softwares that can do these statistics, the GraphPad prizm has this capability.

Basically you convert the values of dilutions into their log values and plot a curve between log dilutions vs response (OD values) and then apply the 4parameter logistic model, which gives you the EC50 values.

This EC50 value is considered as ELISA titer.

Best

Sudeep

Try using IEDB database

this link is useful

https://onlinelibrary.wiley.com/doi/full/10.1002/cyto.a.20377

regards