flash is, imo, a lot trickier to set a rule around. it really is dependant on how you want the shot to turn out.
in low light, if you set to iso 100, 1/50 sec, flash on, the background scene will be dark/near black. there is a certain look some ppl like about the way that is shot but be wary of blowing out your subject unless you have a diffuser.
in low light, my preference with flash is to iso 100, 1/15, f1.8. this opens the shutter longer allowing the flash to freeze my subject while allowing the slow shutter to show up in a nicely lit ambient mood. as you move to iso 200++, this allows your shutter to be set faster without motion blur if your subject moves while still pulling up the ambiance. But that is entirely up to you as the slower shutter opens up some veryy creative motion blur with flash photography.
flash photography isnt as easy to "can" in respect to settings so while my rule may work for me, you may find it is inappropriate to your stylr or taste.