What kind of online calculator I need to determine C on this problem?

Rough sketch:

https://drive.google.com/file/d/1Fleqf4ifPyo7R8YNNatIenE7lH0_JVew/view?usp=sharing

So Point A is where my Center speaker is.
Point B is where I am sitting (in this case 10')
The angle (15º on this example) is the off-axis coverage of the center speaker.

What I am looking is for a calculator that lets me figure out how much total coverage of the audio I will get (C) as the angle varies and also as the distance from A to B varies.Hope that makes sense. :)

Basically some speakers have bad off-axis response so say anything 15º off center, the audio is going to change for the listener so I want to be able to predict how this may affect or not affect a person since of course, it will vary depending on how far they are seating from A.
So if you can point me to an online calculator I could use (cause I am sure I wont get it if you try to explain it lol), that would be perfect!

Thanks!