Explain Kenko 25mm tube Magnification

Hi guys and girls.   Just received 100L and am very excited about using it.   Here is my question.

I have 5DM3 and T2i.   I have Kenko 25mm tube.   I also have Raynox-250.     I used Kenko tube before with 5DM2 and 7D and 70-200 F/2.8 IS II and 17-40L.    I got the best results with 7D, 70-200, and the 25mm tube when shooting dragonflies.

70-200 on 7D

With 17-40L and 7D and the 25mm tube I achieved yet better result

7D/17-40 with a tube

WIth my Sony NEX at Raynox 250, I could get closer:

NEX7/18-55/Raynox

With my current setup, what is my magnification with 100L + 25mm on a 5DM3 body?  What about the same setup on 100L + 25mm  on  a 1.6 body (t2i).   Is it advisable to add the Raynox on the top of that entire setup, or will I lose too much light?

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