Hey @anarayan_das
that is a good question pal, there is no formula it just takes your engineering judgment into it. as we know in fluid mechanics flows required a certain flow distance from the flow inlet to be fully developed. that’s why we maintain our area of interest reasonably far from our inlet this also work in downstream too. just try your best to avoid fluid interaction with the bounding box. you will get the sense of it through experience. for more @jousefm and @1318980 will get back to you.