You can get into extension compensation pretty deeply if you need to.
I try to make it as simple as my simple mind can deal with.
If the lens is extended to twice the focal length, two stops more light is needed.
If the total extension is one and a half focal lengths, only one more stop is needed.
For my 150mm lense, each additional inch of extension requires one third of a stop more light.
For my 20mm, the correstion is (about) one quarter stop for each inch.
This keeps it easy enough to do in my head while working on location. For critical close-up shots, I may get out the tape and calculator.