You can not show this on next row as you want on same sheet. what you can do is you can create this calculation on another sheet and then on Dashboard you can align it with your original sheet.
% metric B = sum(sum total of metric B)/sum(metric A )
% metric C = sum(sum total of metric C)/sum(metric A )
Thanks Ankit.. but the problem here is that the report height may vary from time to time depending upon the data..Is there a way such that the report height increases or decreases on it own without affecting the row heights in the reports?