As you've noticed, dimension TA moves the attachement point for the rack up and down the seat stays. Dimension TB offsets the attachment point perpendicular to the seatstays. If you want to locate the rack with respect to the seat tube instead of the seat stay, you can still use dimensions TA and TB to locate the attachment point where you want it (relative to the seat stay) and then create user dimensions to display the distance to the seat tube.

The bottom of the rack is always attached to the eyelets in the rear dropouts. If you define your dropout by parameters, you can define where this eyelet is with a radial dimension and an angular dimension. If you are using a one of the dropouts that is stored as a static drawing, the rack will automatically be attached to the eyelet if one is built into the dropout. If there are no eyelets in your dropout, then BikeCAD will use the last radial and angular dimension stored in the "Define by parameters" section of the dropouts dialog box. This will seem like a bit of a hack, because you have to set the eyelet position in the Define by parameters section first and then go back to the Define by static drawing section to select a dropout that does not have eyelets.

Having said all this, perhaps the most flexible way to design a rack is to use the newly added option to add extra tubes which can now be attached to a single tube. With this new option, you can build up any kind of rack you like. This is discussed at: bikecad.ca/extra_tubes_single_connection.