Do you mean percentage contribution to the whole amount?
If so, you would have a dimension, such as Store, in a table and then have an expression like this:
sum(Sales) / sum(TOTAL Sales)
You would then need to use the Number format to tell Sense that this is a percentage.
If you want a Margin % it would be something like;
(sum(Sales) - sum(Cost)) / sum(Sales)
Hope that helps point you in the right direction?