Head tube bottom is a common dimension used with framebuilding fixtures to locate the bottom of the head tube relative to the bottom bracket. A few fixtures have their own variation on this dimension and BikeCAD has the option to display them all.
Head tube bottom
Head Tube Bottom is the distance from the bottom bracket to the bottom of the head tube. BikeCAD Pro presents this distance in X and Y components.
The Henry James jig requires a couple of other dimensions that are related to Head Tube Bottom. Head Tube Bottom (Henry James) is the dimension shown below. It is just like Head Tube Bottom except it is measured in line with the steerer tube.