You cannot convert the PSI reading on a welding gauge to LPM on a medical rig. Liters per minute is flow, PSI is pressure and they are not interchangeable like degrees C and F. Keep in mind that people using demand type regulators don’t know how many LPM they are getting. They just know that it’s “enough.”
More on this in the link I referenced earlier: Multimedia File Viewing and Clickable Links are available for Registered Members only!! You need to

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Fortunately, you don’t need to care because
a welding regulator can give you as much or as little flow that you want. Simply adjust it to keep pace with your breathing –
that’s it. Start by turning the regulator all the way down (CCW) then open the tank valve. Gradually turn the regulator CW until you get enough to breath in/out very deeply without wasting a bunch. (Depending upon how deeply you can do it, you will see between 2-7 PSI on the output gauge)
The other gauge is the pressure in the tank. A full tank will be in the 2200 to 2600 PSI range depending what type it is.
Also from another page I referred you to earlier: Multimedia File Viewing and Clickable Links are available for Registered Members only!! You need to

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you can see regulators with pictures showing the difference between the fittings on E size medical tanks (CGA-870) and large medical tanks (CGA-540).
It is important to note that ALL welding tanks use the same fitting as large medical tanks. This page shows what you need to use a welding regulator on a small E size tank: Multimedia File Viewing and Clickable Links are available for Registered Members only!! You need to

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Recap:
1) ALL welding tanks use the identical CGA-540 fittings as large medical tanks like the M, K &H
2) Most small medical tanks of E size and smaller, use a very different connection: CGA-870
There are adapters to interchange the two, but you will have to order them online. You can also buy a special hose kit to fill small tanks from a large one.
All of the above is discussed in more detail on several pages in the links above.
What did I miss?
Marc
Edit to add: The regulator in your picture is a CGA-870 type. The gauge on that regulator shows only what is in the tank.
On the left end of that regulator is a dial that you turn to change flow rate in LPM. There are markings on the dial that you turn to set the flow rate. As you have found, the really common one's are 0-8 LPM. But, 0-15 LPM and 0-25 LPM are readily available online.