calling/using a sql server scalar valued function

I have a button on the main form for submitting the information. But,
part of the process is to see if the user exceeded their purchase
limits. The DBA has provided a scalar-valued function that returns
the information based on the userid that is passed into it. Below is
the if statement line that would use it. The rest of the IF works
when I put numbers in for testing. Now it is time to go to the
sqlserver for the info.

If Me.Subtotal.Value > "scalar-valued function result goes here" Then

Never had to use/call one of these before. How does one go about
calling a scalar-valued function in access07 to get the return value
which is a decimal?

Thanks.
John