Your question is somewhat open to interpretation so I'll put it this way:
you cannot alter report design in any view other than design view, but in design view, no values are available.
If you can clarify where the calculation is - query or table that the report is based on or in the report textbox itself? Maybe some examples of results and the expression that creates those results would help.
Yes, Mod function is available in vba. Pretty sure you can use it in a calculated control; just enter it directly and forget about the expression builder perhaps.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.