How to calculate coefficient of drag for a particular shape?
There are essentially only two ways to do this. 

First you can build a wind tunnel, run experiments and measure the forces involved.

The second is to run a computer simulation. There is software that will do this, but it is complex professional grade engineering software. Free applications are available, but I can't vouch for their effectiveness. You can search for "computational fluid dynamics software" to find options. Adding "free" to that will find you a few cost-free options.

Be aware that setting up and using this software is a complex task that involves creating a CAD model of your shape. You can definitely do the calculation if you are interested, but it's not a simple or quick task.