Can I create a calculation using the grand total from a table view

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Hi All, i have created a table which is giving me average year working hours available and the total sickness in hours for an employee. the grand total is then correct at the bottom of the table. The next thing I need to do is a calculation of 'sickness hours/available working hours' and this will give me my % time lost. but I can`t do it within table as when I do this it only calculates it for everyone who has had a sickness. Is it possible in OTBI to a calculation using the grand total of the two different