Setup a race/course in BBS that you have done with your information. Then adjust CdA in BBS until you get to what your time/split was. Use that CdA for the future races. You might need to run different several prior races/courses just to make sure your CdA estimate is relatively accurate.
Another option is go ride 20-30km in full race setup and import that bike file into the BBS aero analyzer tool. That will estimate your CdA.
I can’t remember for sure but some of these features might require the paid version.
I’ve found it pretty accurate. The way I “figured†out my Cda was to plug in past races then adjust cda until I got close to the time. Then I’d add other races using that cda to see if the time was close.
It has been pretty spot on for me except for one race. But that was because I started in a late wave and was able to sling shot basically the whole ride.