I totally agree with IFRs - you should have an understanding of the tank or vessel code you are using for design, and the best way to achieve that is to develop some spreadsheets, or preferably something in Mathcad if you have it, based on the code equations. As a beginner, you definitely need to start with the code.
Proprietary software is fine, but you need to completely understand the data required to be input, and the meaning of the results. Sure, you'll likely get an answer, but this will be dependent on how well you have driven the program.
(I should point out that I haven't used Etank or Tank, but just rely on my evolving spreadsheet to check fabricator's designs. If I had to formally present calculations, then I would use Mathcad).