Thanks a lot Jim, but can you explain why I have to wrap every calculation in SUM() function? I still don't know tableau so much.
When you bring measures to the canvas they are aggregated - (i.e. after filters are applied the values the remain (by dimension) are aggregated -
your values for current and previous week sales are disaggregated and will be aggregated as you bring them to the viz - that is equivalent to doing the percent calculation at the record level then aggregate individual results when you bring it to the vis - rather than summing the individual values and then determining the percent
Glad to help oput