You can also check belt tension by plinking it like a guitar string and checking the frequency it vibrates at. Need to measure the length over which it can swing freely and weigh the new belt before installation (total length is on the packaging). That gives you the frequency to tune the belt to. A guitar tuning app on the phone is sufficient.
That's how I did it and it worked well. Can't find my notes with the calculation now, but that's how they do it in industrial and automotive applications too.
The twist method is ok for a basic sanity check, but very inaccurate.