The first thing I do when using flash is to look at the flash guide number to subject distance. It is assumed that you are using flash because the ambient light does not provide a satisfactory exposure.You therefore need to make sure your lash, either as a fill in or the main light provides adequate illumination since it shoud reach your subject and should not blow out the subject.With flash, only the aperture matters After this  , then determine the ISO that will not introduce noise and you are good to go

The first thing I do when using flash is to look at the flash guide number to subject distance. It is assumed that you are using flash because the ambient light does not provide a satisfactory exposure. You therefore need to make sure your flash, either as a fill in or the main light, provides adequate illumination since it should reach your subject and should not blow out the subject.

With flash, only the aperture matters.

After this, then determine the ISO that will not introduce noise and you are good to go.