You don’t really need to do any math and can figure out the required length in 30 seconds:
Start off with two parts, those being your centerpoint (in my example, the one on the right) and your column (in my example, the one on the left)
Rotate the column by the desired angle around the centerpoint using F3X/SBS pivot rotation (generally 10, 15, or 30 degrees, something that will add up to 90)
Do the same in the opposite direction
Now you have the correct distance and the wall can connect seamlessly all around.
If you require a shorter or longer wall, all you have to do is move those column parts inward or outward by their local axis.
Alternatively, you can start off with the wall part at the desired length, (useful if you don’t want those in-between column parts) and move them locally until they’re close to connecting. Then, just patch any small gap with resizeallign:
https://gyazo.com/40166bf2fc91ebd81584dbf2f2f4b8b7
If you want more info on the topic of curvature, I wrote a guide here: FP1's Guide to Building Curvature in Studio