Mamiya 120 Macro lens

I just purchased a used Mamiya 120 macro lens for use on my 645. It arrived
without the instruction sheet. I have checked Google, Mamiya's pages, and
Photo Net.

I plan on shooting small reptiles in a studio setting with flash.

I need to ask if anyone can tell me the f-stop correction factor when the
magnification is at 1:1, 1:2, 1:5, etc.

One message that I found on Photo Net indicated that at 1:1 I would have to
add 2.5 stops and that at 1:2 I would have to add 2 stops to my light meter
reading. Can someone confirm the settings/adjustments that I need please?

Thanks and good shooting.
Jack

Jack Milchanowski