As described that situation indicates an EI of 3200, if I did the math right.
What we don't know is how fancy the OPs flash unit is.
If it has an automatic mode like Nikon's "A" mode, the flash may be able to adjust its exposure to match the EI and aperture with no extra work.
If its a studio strobe it can be adjusted to exactly what is needed and tested with the flash meter.
If its an old flash unit, moving it closer or farther from the subject may be the only choice.