Gordon,
A Worm/Worm-Gear set is, essentially, a Trapezoid Thread screw driving (or, occasionally, being driven by) a Spiral Spur Gear. In a Spur Gear the OD of the gear is the Pitch Diameter + 2/Diametral Pitch of the gear. Thus, the driven area of a Spur Gear is 4/Diametral Pitch. The Root Diameter of said Spur Gear has an additional clearance of .3125/[Diametral Pitch[/i] below that (Root Diameter = Pitch Diameter - 2.3125/Diametral Pitch). The Pressure Angle of the Involute Tooth you are using will be half the included angle of your mating Trapezoid Thread.
The Pitch Diameter of the Trapezoid Thread should be located 2/Diametral Pitch below the Major Diameter of the Worm. The Minor Diameter of the Worm should (in classical definition) be located 2.25/Diametral Pitch below the Pitch Diameter giving you a thread depth of 4.25/Diametral Pitch for your Trapezoidal Thread. [You really want to use wires to measure the Pitch Diameter and Pitch of your Trapezoidal Thread.]
The Tooth Width on your Spur Gear is Pitch Diameter*sin(180/Number of Teeth (+0/-Clearance) on the Pitch Diameter. This is also the Thread Width and Thread Spacing (though the Spacing tolerance will be +Clearance/-0) to define the per Revolution Pitch from which the Spiral Angle for the mating Spur Gear may be calculated.
Does this help?