How to calculate the optimum tilt angle for a solar panels?

I have been going research on how solar panel’s tilt angle affects their efficiency. I have read some papers on the calculation of solar irradiance on tilted surfaces (https://doi.org/10.1080/07055900.1986.9649238) but have not been able to find any steps on how the optimum angle is actually calculated. The main problem is that there seem to be different models on how diffuse solar radiation in estimated and I was wondering is someone knew which kind of model best approximates solar radiation on titled surfaces from tropical regions like southern India.