This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The width of the door should be about 3/16 in. less than the width between the jambs, allowing a clearance of about 3/32 in. on each side, and the opening edge should be slightly beveled. The standard bevel to which locks are made is 1/8 in. in 2 1/4 in., but where the door is narrow, it may require the lock-face beveled to as much as 1/4 in. in 2 in., or even more. An equal clearance should be left at the upper rail, while the bottom rail may require from 1/4 in. to 1/2 in., in order that the door may swing clear of the floor covering. Where "saddles" are used - which are not to be recommended - the door may fit within 3/32 in. An appreciable amount of the clearance will be taken up by the layers of paint or varnish.
A simple rule for finding the width of butt required for any door is: To twice the thickness of the door, less 1/2 in., add the greatest amount of projection of any part of the door casing beyond the face line of the door (which is usually the base block). Thus, if a door is 2 in. thick, and the base block projects 1 7/8 in., the width of the butt will be 2 in. + 2 in. - 1/2 in. + 1 7/8 in., or 5 3/8 in., in which case a 5 1/2" width would be used. The edge of the butt will thus be kept 1/4 in. back from the face of the door. One-half of the thickness of the butt should be cut out of the door, and one-half out of the jamb of the frame. In locating the butts, it is usual to keep the lower end of the lower butt in line with the upper edge of the lower rail, while the top end of the upper butt may be kept from 6 to 7 in. below the upper edge of the top rail. Where three butts are used, it is well to keep the lower end of the intermediate butt in line with the upper edge of the lock-rail, instead of placing the butt midway between the upper and lower butts, as the butts will then line up with the framing. By keeping the pin of the lower butt slightly in advance of the upper butt, the door, in opening, will rise at the toe and increase the clearance, so that inequalities in the floor level may be overcome. For first-class working doors the following conditions must be observed: first, the floor must be level in every direction; second, both jambs of the door frame must be accurately plumbed, facewise and edgewise, else the toe of the door will either rise above or fall towards the floor when operated; third, the head-jamb, or transom, as the case may be, must be level; fourth, the butts must be of good quality, well fitted, and the pins kept true to line.