I use a 6-1/2-ft. spirit level to plumb and straighten interior door jambs. In order to leave both hands free to handle the shims and to drive nails, I wedge the level against one of the jambs to hold it in place. This lets me monitor the jamb continually as I make fine adjustments with the shims, rather than having to check and recheck the alignment with a hand-held level.
Once I’ve assembled all the jambs for the house, I cut a spacer the exact width of the door opening out of 1×6 or jamb offcuts. As shown in the drawing, the spacer rests on the floor between the jambs, maintaining the correct dimension at the bottom of the door opening.
Spacer in place, I temporarily shim the jamb in its rough opening. Then I wedge the level against one of the jambs with scrap pieces of door stop. The spaces above and below the level should be equal. Now I can plumb and straighten the jamb in the usual manner, tapping shims in and out until the jamb is straight up and down and true as an arrow. Then I nail the jamb to the jack studs with a pair of 10d finish nails through each set of shims. After double checking for alignment and making any necessary adjustments, I repeat the procedure on the opposite jamb.
D. B. Lovingood, Portsmouth, VA