Did you checked all the options available in Dimension Limits? With this option we can achieve most things very easily. With Calculated Dimension and Expressions it is bit difficult to achieve this things.
To make the Top N charts easily Dimension Limits is provided, if we have many more options other than top N, like Show Others, Show Totals, Show First N, Smallest N, Largest N, Show only Values that are >, <, >=, <= N%, Show Values Accumulate to N%.
Try to do all the options in Dimension Limits using Calc. Dimension and Expressions then you came to know how easier the Dimension Limits is.
