Corrosion allowance is not something you calculate. It all depends on you process medium, your material of construction, your temperature, and industry experience with these conditions. Provide these and we'll be able to assist.
It's not a calculation but sort of a rule of thumb.
Start with looking at the difference between the minimum retirement thickness and the minimum thickness of the tank plate for construction so that you don't add steel on unnecessarily. Some standards give you a minimum corrosion allowance and you will have to make a call on whether that is sufficient based on expected corrosion rate and the corrosion control measures such as lining and cathodic protection. EEMUA 183 points out issues of low cycle fatigue in the annular section and recommends keeping it as thin as possible, suggesting a maximum corrosion allowance of 2mm for plate not exceeding 12.5 mm, and 1mm for thicker plate. Also, bear in mind that the usual corrosion morphology is pitting for which increased corrosion allowance doesn't offer a reliable barrier.
