If you are draping a garment like coat or jacket, it is easy to over-fit it as you drape against the dress form. To keep the garment from being too tight and needing alteration right off the bat, pad the dress form with a heavy sweater, jacket, or coat.
This will give you the extra volume you need to play around on the dress form and give you a better result without having to figure the ease geometrically after the garment is draped and you are correcting the pattern.
As a side note, remember as you are fitting the sample on the person to use the same sweater, coat, or jacket so that you don’t create ease just to remove, just to find you have to add it back.