F-stop is a theoretical number based on the calculation you describe. A T-stop is a measured value for the light hitting the sensor normalized back to be equivalent to a f-stop. Since I am using DXOMark numbers, here is there definition:

https://www.dxomark.com/dxomark-lens.../#transmission

But a few other references:


https://www.youtube.com/watch?v=DzSlQRewS7Q
https://www.youtube.com/watch?v=jYRJVRMlIe8