Help Me Understand Shooting With The 140mm "Macro"

I've just returned to my RB67 Pro SD after a couple of years' lay off, and need a little help setting the correct exposure and focus with the Macro 140mm lens. I have the No.1 Extension Tube fitted.

I'm sitting at my desk, focussing the lens on bubbles in a glass of water about 18 inches away. This is what I'm doing:

1. Set the Green Scale to 20
2. General Focus, followed by magnifier
3. Turn to the diagram on the side of the body, and read the scale along the top, the one that goes 10, 20, 30, etc. I get a reading of 24. I adjust the Green Scale to 24.
4. Set my aperture to f/4.5
5. Now it's time to set the exposure time. I take my digital camera, a Canon PowerShot G10, set it to ASA 400 for the film speed, and 4.5 for the aperture. I take a shot, but it's too bright, so I dial in -2 EV. Now it look just right for my purposes. This now gives me a reading of 1/30.
6. I set the exposure wheel on the RB lens to 30
7. I check the diagram on the side of the RB gain, this time the scale at the bottom, which reads +0.5
8. I turn the STEP scale to 0.5

Am I now set to pull the trigger?

What is nagging at my mind is the +0.5. Should I change the exposure time from 30 to 15 to compensate? Or half way between 30 and 15?

I have an old Weston light meter, and a couple of apps, none of which fully agree with each other. I don't want to spend hundreds of pounds on a Seconic. So, the digital camera it is. It gives me a reading, but also allows me to see the effects of any changes I make, such as the EV comp.

Any advice gratefully received.

Rob, not sure you've got the 140mm user giude PDF, but here is a link to it. The guide does go over the Exposure Compensation you've mentioned.
Mamiya-Sekor_Macro_C_140mm_f4.5.pdf

I get about 90% of what you're asking and stating, but a bit fuzzy on the actual exposure compensation. Obviously with the extension tube and the bellows extension there is light loss. With that said it would take me some more thought on how to figure that out exactly from your comments. As I have the same setup as you've mentioned with the ProSD.....etc. however, I have the Prism II meter that gives me the information directly through the entire lens, tube, bellows, filters, etc. So, the compensation I might do is bracket a bit and nothing more from the Prism II reading either set for Average or Spot metering.
You're way ahead of me in figuring things out with using any type of external meter.

Please let me know if this helps at all using the guide and such. Good luck !!!

Thanks, Trail. Bracketing is a good idea, but with only ten shots on a film, that can start getting expensive and wasteful to a Scotsman!

Actually, the answer is right there, last paragraph, page 3 of the Mamiya guide: "Set the shutter speed to one step lower". I'd read that before, without really registering it. Thanks for pointing it out to me again!

Good to hear, Rob. Glad I could be of help......enjoy your macro work outings.