I LOVE this little gaiwan from Shawn McGuire (formerly Greenwood Studios, now Great Wheel Studio on Etsy
), because the flare is both wide and the lid set deep below the rim. PERFECT. A and B are both about 1 cm.
Plus it is gorgeous and goes wonderfully with these cups, better than the gaiwan that I bought at the same time, which has a reasonably deep B but was a bit too narrow in A, and often scorched my fingers. But it had a saucer and I worked out how to use it comfortably before it sadly was shattered (one of many victims of a tiled kitchen floor, never again!).
I have a bunch of these cheap ($4.99) gaiwans purchased at my local brick n' mortar shop in Chinatown, where A & B are more like 7mm, but using the grip I show here, making use of the saucer, I rarely scorch my fingers. Using them without the saucer, sometimes I get a scorch.
Many of the lovely shapely and aesthetically exciting gaiwans I see posted here and elsewhere look like A and B are both quite small, and it is that which makes me distrust them for my fingers. It obviously depends on the actual size of the gaiwan, and a larger one may have a proportionally smaller rim whose actual measurements are quite sufficient. I mostly use my inexpensive gaiwans for comparative tasting sessions, where the uniformity of their mass-produced design is more important than the details of their construction, but I am wary of buying any finer and artisanal pieces with such small rims, because I know I won't use them as much as Shawn's little gem.
Your finger-scorch tolerances and preferred minimum A and B may vary, of course!
I find my finger get scorched when either or both A and B are too small. A narrow rim beyond the gaiwan lid (A too small) , even if the lid fit is low inside the rim, isn't enough distance for my sometimes shaky fingers. Similarly, if the lid fit is shallow inside the rim (B too small), even a wide flare doesn't prevent scorching. I like to see both A and B be generous, so the tea level (the red scalloped line) stays away from my fingers as I manipulate the gaiwan.