If the newborn babies diaper is dry and not wet or poopy then there's no need to really subtract any weight because the diaper when dry doesn't weigh very much at all to register on the scale.
The dry diaper barely weighs an ounce but a weight diaper may weigh 1/2 lb or a 1 lb when fully wet.
If the babies diaper is partially wet and you weigh the baby I would subtract a 1/2 lb to get the best accurate weight or if possible just remove the newborns diaper for a little bit until you weigh the baby.
Some doctors will have you remove the babies diaper before weighing them to get a proper weight.
That's what my doctor had me do with my baby is to have the diaper removed and then put it back on when the weighing was done.
With an older toddler we just stripped them down to their diaper and then if the diaper was wet I'd put a new diaper on the toddler and then the weight was more accurate.