It is easy to say I'm using a bath volume of 100mL but you need to try it. Will it work? There needs to be enough volume that the fiber is completely submerged when resting in the dye bath. Any part of the fiber that protrudes above the bath can end up lighter than the rest; it can only dye in the dye bath.
You have conflicting demands here; you want to keep the volume as small as possible to minimize the dye reacting with water yet you need enough volume to submerge and move the goods you are dyeing. You might have to do a tiny experiment of measuring different amounts of water into your jar and seeing what works for your substrate. I don't know what the answer is, depends on the shape of the jar, the fiber package etc. I suspect that 100mL is too little for skeins and that more than 250mL is too much water for the dye. Part of what you will have to decide.