Read only, calculated fields

Hi, I am relatively new to creating forms and I am probably making a rookie error here. I have a series of numerical fields and at the end of the series I have a field which is simply a sum of the fields.  I have set this calculated field as "Read Only", but when I press Tab from the final numerical field, the focus still goes to the calculated field (but doesn't allow entry).  As it is read only and calculated, I would't expect it to get the focus from the tab key.  Hope this makes sense.