To have the rear end of your top tube attach to the seat tube about a foot and a half from the BB, simply change the seat tube length (c-c) measurement shown below in blue. At this time, be sure to note the difference between the seat tube center to center measurement and center to top measurement. Although this dimension is meant to be entered in millimeters, be aware that you can also type in mathematical expressions. This is a convenient way to handle the conversion from inches. For example, a foot and a half is 18 inches. Therefore, by inputting 18 * 25.4 BikeCAD would convert this to 457.2 mm. You may, at the same time, wish to adjust the amount of seat tube which extends above the top tube. This input field is shown below in red. Also, how the rest of the frame will respond to changes in seat tube length is going to depend on what option is selected in the menu shown below in orange. For more on that, see this link.