While BikeCAD does give you the option to directly input chainstay length, for some, the concern isn't so much exactly how long the chainstays are, but rather how much of a gap is there between the rear wheel and the seat tube. For that, we have the option to control the rear end by rear wheel gap. This has been an option in BikeCAD for a long time. However, one deficiency of this option was that it would not take into account bent seat tubes. In BikeCAD version 16, this issue has been resolved. BikeCAD will correctly set the chainstay length to maintain the minimum specified gap between the rear wheel and the nearest point along the back edge of the bent seat tube and bottom bracket shell.
If you are reading in an older BikeCAD file which specified rear end geometry by rear wheel gap but also featured a bent seat tube, you'll find when that file gets read into BikeCAD version 16, the geometry will be identical to how it appeared in earlier versions of BikeCAD, but the menu will have switched to controlling the frame by chainstay length. You can switch the menu back to rear wheel gap, but when you do, it will reflect the actual rear wheel gap, not the dimension that measured to an imaginary straight seat tube.