Power pivot - calculating market share per segment

Perhaps this will help.
1. Produce a second table where distinct country names are listed and then cumulative stats are calculated using formulas like: =SUMIFS(Table1[Value Sales Year Ago],Table1[[Country]:[Country]],$H2)
2. Add the second table to the data model
3. Make a 1 to many connection between the two tables in the data model
4. Modify the Total value Year ago function to read: =sum(Table1[Value Sales Year Ago])/SUM(Table2[Value Sales Year Ago])
5. As modeled on the Pivot Table 2 sheet the function now displays the disired results
Let us know if you have any questions.