1.1 - 1.2 MM.....such numbers tell me absolutely nothing.
I'm stuck in the last century.
A B or OB with any German pen with a semi-flex will be like a fat M, a good writing nib not a signature nib.
I had my fat and blobby BB on my 605 made into a 1.0 stubb (even that told me less than I know...knowing nothing much figured 1.0 would give me a thin 'vintage' B width. ), as far as I can see the semi-flex B I had on it, was thinner.
An OB is easier to find the sweet spot....an OM or OF requires more precise nib placement.
First you have to know what grind angle your oblique has 15 or 30 degrees....and that will be pure luck.....I have in a mix of semi-flex and maxi-semi-flex (that too pure luck unless an Osmia) some 13 obliques...In OBB, OB, OM and OF I have one 30 degree grind to match the 'normal' 15 degree grind.
So until you get use to your oblique, use the clip as a sight. For a nib with 15 degrees, aim the clip as you post the cap, in the middle between the slit and the right edge of the nib. That will place the nib exactly right on the paper.
If your nib is 30 degrees of grind, align the clip so it's in line with the right hand edge of the nib, then put the nib to paper and write.
Opps.....do re-grasp the pen after you've aimed the clip....don't look at the nib....which will be canted, then just write. Shortly you won't need to re-grasp the pen because you will just grasp the pen with the nib canted.
Vintage and semi-vintage are at least 1/2 size narrower than modern....out side the 200 which remains true to size of the other eras.
It all depends on what sort of paper you are forced to play with..........poorer paper I can see an F or OF nib, if you hand is anywhere near light............then use a dry ink, Pelikan.....in there will be some nib width spread.
A good poster here said, stub and CI was 100% line variation, semi-flex was line variation on demand....................................and I see it as getting some of that old fashioned fountain pen flair with out having to do anything.
Atkin Lambert dip pen...should be superflex at least....wet noodle should be uncooked vs that.
Regular flex, semi-flex and maxi-semi-flex are all in a 3 X tine spread set..........trying for more = $$$ nib repair.
Mash a regular flex nibbed pen so you get the 3X max vs a light down stroke.
That is the basis of my system....the starting point. Nails and semi-nails lack tine spread to worry about and even semi-nail can be something for the heavy handed.
Semi-flex takes half of that pressure to reach 3X.
Maxi-semi-flex half of that or 1/4th the pressure needed to mash a regular flex to 3X.
Superflex....(4, 5&6 or the rare 7X tine spread)..........first stage..Easy Full Flex 1/8th (have 5-6), wet noodle 1/16th (have 2&3rd should be here on Monday), Weak Kneed Wet noodle even less...I have none of them.
I have Hunt 99-100-101.........that makes a wet noodle look like it was recovered uncooked and fossilized from Pompey.
Do have someone sew a shirt pocket a bit closed...as a pen pocket, or carry it in a leather holster, and any '50-65....140, 400-400nn will do just fine. Any pen dropping will be damaged. Lands End will sew a pen pocket into a shirt if you ask...........that prevents it from every falling out if one bends over..........there should be closer owners of a sewing machine than Land's End.