Where should calculated fields be created and why?

At the data source level in almost all cases. They are then available to every chart and every report using that source, and there is one definition to maintain.

Chart-level calculated fields are invisible to everything else, which is how you end up with five slightly different versions of the same metric.

One thing to get right: for a rate, divide the sums rather than averaging row-level ratios. SUM(Revenue)/SUM(Sessions) weights correctly; SUM(Revenue/Sessions) treats a one-session row the same as a thousand-session row.