Well, I'll go to the foot of our stairs!
I have just come out of the workshop for a well earned brew after making 4 adaptors for the float bowl drains and just saw your post! I had a die of the correct size and some hex brass rod, so no big deal really. I was surprised that the drain screws were not listed separately in my parts book - perhaps they were never available? I have a few spare sets of carbs and could have adapted (butchered!) a set but if they are made from unobtainium, best to make my own.
Anyway, you have set your floats at 22mm and that has given you a level about 3mm down from the joint face - have you run it at that without the flat spot?
I'm still finding disagreements about whether they should be 22 or 24 mm and I'm about to do my spare 400 but my 350/4 runs fine on 24mm.
It's strange how these carbs seem finicky on float height - my old man used to switch the fuel off on our BSA Golden Flash outfit at the top of the road and it ran well on a diminishing float height right to the house - ah, for a simple life!
Ian