Is it a label or a textbox? If the form is open, the report should be able
to refer to the control on the form. If the form isn't open, you will need
to rerun the calculation in the report or pass the value to the report as
you open it.
To refer to a control on the form:
Forms!FormName!ControlName
This will get the Value property from the control, it you are using a label,
it doesn't have a value. Instead, you'll need to use the Caption property.
Forms!FormName!ControlName.Caption
To pass the value to the report you can use the OpenArgs argument of
DoCmd.OpenReport, depending on your version of Access, or you can use a
"global variable" to hold the value then refer to that variable in the
report.