How to calculate % of a total

[#1]

I am trying to create a calculation that gives me this year % of business. How would I do that using the example below:

Brand	Customer	$ Amount	% of Business
XYZ	Customer A	50	0.31
XYZ	Customer B	25	0.16
XYZ	Customer C	35	0.22
XYZ	Customer D	50	0.31
Sum XYZ		160

I'm trying to figure out how to use an expression to calculate the last column. It has to be able to be dynamic based on the date/year selected.

Thanks!

[#2]

Sum(Amount) / Sum(Total Amount)