I’ve been reading about the differences between T-stops which are based on the measured amount of light as opposed to a calculated f/stop, and I’ve found this wikipedia page to be super useful.  Wikipedia.org page on F-numbers

Check it out as it has lots of great information including all the math formulas and other useful information  – and btw   T-stops take in account the efficiency of the lens and can be different from a calculated f/stop because most lesses only transmit about 90 or 95% of the light and sometimes even lower – down to about 60%.