Are you blanketing your methanol tank or are you really venting your methanol tank? The last API 2000 version I have is the 5th Edition (1998) and it is titled "Venting Atmospheric and Low-Pressure Storage Tanks". Unless the API has changed the scope of the application in API 2000, the subject is still a venting operation - which is entirely different from a blanketing one. It is very important to use the correct engineering terms in our Forums. Otherwise, a lot of confusion and erroneous information will be transferred. A blanketing application is a batch, static application of inert gas involving a volume of inert gas. A venting application is a continuous outflow of gas or vapors and it is a flow rate process.
API 2000, I believe, is dedicated to safely venting built-up vapors and gases within storage tanks in order to avoid a hazardous over-pressure in the same tank. Blanketing a tank with an inert gas - such as nitrogen - involves establishing a static volume of inert gas over the internally stored liquid. This static "blanket" volume will be evacuated (vented) or replaced with any increase or decrease of liquid inventory. The volume of inert gas employed or used is dependent on the movement of the stored liquid and, to a less degree, on the ambient (or other) temperature affecting the gas blanket. I don't think this is what you have as an application.
Please clarify or correct your initial query and furnish a sketch of your application as well as submitting a copy of your detailed calculations with copy of the referenced API 2000 instructions for our members to follow and apply to your calculations. I am sure that most of our members, like myself, are not financially set to maintain a complete, up-to-date API library of the latest editions or versions.
Once you've submitted your materials (preferably in a spreadsheet workbook), I am sure our experienced members will pitch in to answer your query and offer their valuable advice.