To show each region's share of sales, every row needs two numbers: the region's sales and the sales of all regions. The second one has to ignore the row's Region filter. REMOVEFILTERS and ALL, used as CALCULATE filters, do that:
All Region Sales = CALCULATE([Total Sales], REMOVEFILTERS(Store[Region]))
% of Total = DIVIDE([Total Sales], [All Region Sales])
Be precise about what you remove. REMOVEFILTERS(Store[Region]) removes only the Region filter, so a Category slicer still applies. ALL(Sales) removes every filter on Sales and on the tables that filter it, slicers included, so the percentages go wrong as soon as a reader slices the page.
ALLEXCEPT removes every filter on a table except the columns you list, which suits a share within a group, such as a product's share of its category.