First, understand what is the "ocean" (e.g. assess the main dimensions of plane, etc.)
Second, understand the unit with which you deal (e.g. ball, ppl, table)
Third, understand the limitations & bottle-necks (e.g. has to be a space for the people to move back & force)
The last, do the math
Important: don't try to be precise. The way you think is much more important than the figures. But don't diminish meaning of figures as well.