Wista Copal Press shutter #1, how to calibrate the f/stop for differents focal length

[Ginette]

I have a very nice Wista Copal Press shutter #1. It came with an aperture scale. Are some data are available for this shutter as an f scale is already affix to? At the top of the scale I can see this indication W180.
Same as this one (http://www.bhphotovideo.com/c/product/63488-REG/Wista_211341_1_Press_Shutter.html). Mine doesn't have the black tube, don't know what it is? See mine below.

For calculate the good f-stop for a specific focal lengh I think that I need to take exact diameter of each f-stop. Focal Length / diameter = new f-stop Right??

EX: Actual 5.6 on the aperture scale = diameter of the blades 25mm. So for a 300mm it will become 300 / 25 = f12 for a 150mm lens it will be 210 /25 = f8,4

Is this a simple way to calculate an f-stop correction that can be enter in an application like the Reciprocity Timer in terms of a final correction of the time. ex: the calculated 8.4 vs the original 5.6, what is the mathematically coorect operation 8.4/5.6 = 1.5 stop less??

Sorry for my very bad English, hope you can understand what I ask about.

[Ginette]

If the scale indication is calibrated for a 180 lens, can I calculate an f-stop factor this way?
for a 360mm, it will be -2 stops
210mm cells will be -1stop 1/6
150mm will be + 1/6 stop

[Ginette]

Can someone help me with theses f-stop calculations?

[John Koehrer]

The second paragraph in the 1st paragraph is correct.......But..........the diameter is measured through the front element then calculated.
It may be easier to just make up a card with the apertures for each focal length.

[Ginette]

And is the second post good too?
On the actual scale I have the indication W180. Is that means that the f-stop scale is calibrated for a 180mm lens??
And then 210mm cells will need 1stop 1/6 compensation (210/180) and a 150mm will be + 1/6 stop more luminous (150/180).
I have Graphic-Kowa 210mm and 150mm cells that will fit parfectly on this shutter.