New column or field

Hi, 

Ive searched through all the fourm and cant find an answer specific to this. 

I have two tables that are joined together in my dataset. Im looking to create a new calculated field that takes one field from one table and divides it by a different field in the other table. To give you an idea..

Table 1 -Field Name:Revenue

Table 2- Field Name: staff

Im trying to divide the two. But it seems to only calculate in one table withjout picking up the second table. I was looking at DAX functions but they didnt show simple functions between wo tables.

OIf there is a link to another questions like this that was answer then let me know. I just cant seem to find one.

Thanks