6w5d newborn is not dehydrated however soft spot feels a little bit sunken. she's alert no other signs of dehydration. how sunken should it be? er?

Not much: If a child is otherwise well, taking normal feeds and urinating routinely without vomiting, diarrhea, fevers, etc, a slight depression of the fontanelle ("soft spot") need not be cause for alarm. Unless something else of concern is happening, this is not a reason to visit the ER. A pediatrician could safely confirm that this is normal during regular hours.

Baby water baloon: The soft spot is just an area between the skull sutures where you can access the waterworks surrounding the outside of the brain.This is somewhat like a water balloon, which should be flat or a little sunken when baby is upright and flat or bulge a little when baby is flat or hanging inverted.A 6 wk old happy baby does not need to go near an ER and get exposed to weird sickness.