Let me know if you have an extra one lying around.
For my 120mm Planar
Let me check, I have a few of them. I know I have a couple of 10s and a few more. How does one estimate the additional exposure for the extension tubes?
If you (like me) are rather lazy, you let something else calculate for you.
Been out of town so....sorry for the delay.
I am not sure on the calculation but the link that Q.G. supplied looks good.