Regulating The Action (page 44)
Leveling The Keys

Level the keyboard by adding or removing punchings        To level the keys to be exactly the same height, add or subtract paper or cardboard punchings under the balance pin felts. If just a few keys are below the level of the others, you may wish to simply remove the offending keys, add or remove a few thin paper punchings and sight the correct height. A decent punching can be made with typing paper for the paper ones and thin cardboard (a business card or matchbook cover) for the cardboard punchings. They need not be perfectly circular, though they must be perfectly flat.
       To accurately level all the white keys use a steel straight edge ruler. Set height of white keys 1/8 inch below top of keyslip, sharps flush with white keytops. To use a straight edge, set the height of the end keys by removing or adding punchings under the balance pin felt so that the bottom of the key front is approximately l/8th of an inch below the top edge of the keyslip. Then set the height of a middle key (D or E above Middle C) in the same manner. Now lay the edge of a yardstick on the heightened keys and add the proper punchings to each key.

Regulating key height       Leveling black keys is done in the same way. The height of black keys should be so that the wooden base below the black top piece is level with the adjacent white keys. Check to ensure that the key does not go below the level of the white keys when depressed and do not bend the thin paper punchings when you set them in position as that will affect the key height. If felt punchings have hardened, rotted, or been eaten by mice, replaced them with new felt.