Congrats on the new T32! We have a couple T32 awards, and this has been our practice: our department pays all of our PhD students a standard rate per hour and guarantees support at 20 hours/week throughout their time in the program (whether it be as a Grad RA, TA, fellowship recipient, etc). Therefore the stipends are calculated in the same manner: our established FY grad rate x 20 hours/week. We have some grad students who become trainees in programs outside of our dept. Their calculations seem to be similar, and if their trainee appointment pays less than our dept grad rate, we ensure their total stipend is at least equal to our rate x 20 hr/week.
Due to the NIH trainee stipend caps, only a portion of their stipend can be charged to the T32 grant and the remainder "supplement" is charged to match funds we've raised in connection to the T32 grant. In other cases, supplements may need to be charged to the advising faculty member's discretionary funds.
Happy to discuss more about T32 post-award management at any time!