You don't cross over from weight to volume, blackpowder charges are always in weight.
To set up your powder tubes, fill the measure with powder. Take some powder and weigh out on your scale, then pour it into the powder tube, and set the tube so that powder charge is level with the top of the tube. Then take the tube and put it up the bottom of the measure, pull the handle until you don't see the powder moving behind the glass, let the handle go, and then remove the tube and weigh the charge. Adjust the tube if necessary, then when it's all set charge the tube, dump into the case, compress the powder and seat the bullet and wad.
Easy cheesy.
A wise man can always be found alone. A weak man can always be found in a crowd.