Help with iif statement in a report

Hello,
If more info is needed just ask...I didn't want to clutter this up w/ too much detail on what the fields are and what I am trying to do.

I'm using Reporting Services. For the y axis on a chart report I have the following:

=round(Fields!CountOfLoginTime.Value/ Sum(Fields!no_of_weeks.Value, "average")/5,0)

This works and returns the expected result.
However for one instance ( a parameter )  I would like to divide by 6 instead of 5.
So I was thinkng I needed an iif statement

I have one parameter in the report... Parameters!location.Value

So how can i write the following iif statement ? or any other method that can be used

if Parameters!location.Value = 'ab123' then
round(Fields!CountOfLoginTime.Value/ Sum(Fields!no_of_weeks.Value, "average")/6,0)
else
round(Fields!CountOfLoginTime.Value/ Sum(Fields!no_of_weeks.Value, "average")/5,0)

Thank you for your time