Absolutely. While commonly used for commercial systems, it is perfectly suited for residential ductwork, especially for complex custom homes with long duct runs and many fittings.
Export your calculations to include in your project’s technical documentation. Conclusion ashrae duct fitting database 6.00 05 free download
You have a 24" x 12" rectangular main duct. A 12" x 8" rectangular branch takes off at 45 degrees. You need the pressure loss for fan sizing. Absolutely